evaluate problems found by staticcheck.

[coocood]

lockstore/lockstore.go:65:16: func (*node).entryLen is unused (U1000)
lockstore/lockstore.go:236:21: func (*MemStore).getNode is unused (U1000)
metrics/metrics.go:99:26: prometheus.Handler is deprecated: Please note the issues described in the doc comment of InstrumentHandler. You might want to consider using promhttp.Handler instead. (SA1019)
pd/client.go:63:2: var errFailInitClusterID is unused (U1000)
rocksdb/filter_builder.go:49:34: should use !bytes.Equal(b.lastWholeKey, key) instead (S1004)
rocksdb/filter_builder.go:74:33: should use !bytes.Equal(b.lastPrefix, prefix) instead (S1004)
rocksdb/options.go:17:2: only the first constant in this group has an explicit type (SA9004)
rocksdb/options.go:41:2: only the first constant in this group has an explicit type (SA9004)
rocksdb/sst_file_iterator.go:171:2: should use 'return ' instead of 'if { return }; return ' (S1008)
tikv/closure_exec.go:267:2: field count is unused (U1000)
tikv/closure_exec.go:268:2: field limit is unused (U1000)
tikv/closure_exec.go:661:6: this value of err is never used (SA4006)
tikv/cop_handler.go:23:5: var dummySlice is unused (U1000)
tikv/cop_handler.go:151:23: func (*evalContext).newRowDecoderForOffsets is unused (U1000)
tikv/cop_handler.go:172:23: func (*evalContext).decodeRelatedColumnVals is unused (U1000)
tikv/cop_handler_test.go:264:22: store.svr.buildDAGExecutor undefined (type *Server has no field or method buildDAGExecutor) (compile)
tikv/dbreader/db_reader.go:124:2: redundant return statement (S1023)
tikv/mvcc/tikv.go:146:3: this value of data is never used (SA4006)
tikv/raftstore/applier.go:79:6: type keyRange is unused (U1000)
tikv/raftstore/applier.go:149:2: field ranges is unused (U1000)
tikv/raftstore/applier.go:263:2: redundant return statement (S1023)
tikv/raftstore/applier.go:530:2: field merged is unused (U1000)
tikv/raftstore/applier.go:540:2: field readySourceRegion is unused (U1000)
tikv/raftstore/applier.go:1036:2: redundant return statement (S1023)
tikv/raftstore/applier.go:1094:2: redundant return statement (S1023)
tikv/raftstore/applier.go:1119:2: redundant return statement (S1023)
tikv/raftstore/applier.go:1159:2: redundant return statement (S1023)
tikv/raftstore/applier.go:1430:2: field merge is unused (U1000)
tikv/raftstore/applier.go:1431:2: field readyToMerge is unused (U1000)
tikv/raftstore/applier.go:1527:19: func (*applier).resumePendingMerge is unused (U1000)
tikv/raftstore/config.go:267:20: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/config.go:289:20: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/config.go:298:20: error strings should not be capitalized (ST1005)
tikv/raftstore/config.go:302:20: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/config.go:311:20: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/fsm_peer.go:87:20: func (*peerFsm).drop is unused (U1000)
tikv/raftstore/fsm_peer.go:91:20: func (*peerFsm).regionID is unused (U1000)
tikv/raftstore/fsm_peer.go:115:20: func (*peerFsm).hasPendingMergeApplyResult is unused (U1000)
tikv/raftstore/fsm_peer.go:232:27: func (*peerFsmDelegate).resumeHandlePendingApplyResult is unused (U1000)
tikv/raftstore/fsm_peer.go:879:27: func (*peerFsmDelegate).validateMergePeer is unused (U1000)
tikv/raftstore/fsm_peer.go:883:27: func (*peerFsmDelegate).scheduleMerge is unused (U1000)
tikv/raftstore/fsm_peer.go:887:27: func (*peerFsmDelegate).rollbackMerge is unused (U1000)
tikv/raftstore/fsm_peer.go:1198:3: should use bytes.Equal(leftKey[:tablecodec.TableSplitKeyLen], rightKey[:tablecodec.TableSplitKeyLen]) instead (S1004)
tikv/raftstore/fsm_peer.go:1409:4: should use 'return ' instead of 'if { return }; return ' (S1008)
tikv/raftstore/fsm_store.go:89:2: field cleanUpSSTScheduler is unused (U1000)
tikv/raftstore/fsm_store.go:109:24: func (*PollContext).flushLocalStats is unused (U1000)
tikv/raftstore/fsm_store.go:170:2: field lastCompactCheckKey is unused (U1000)
tikv/raftstore/fsm_store.go:171:2: field stopped is unused (U1000)
tikv/raftstore/fsm_store.go:249:2: field resolverScheduler is unused (U1000)
tikv/raftstore/fsm_store.go:250:2: field snapScheduler is unused (U1000)
tikv/raftstore/fsm_store.go:918:6: type regionIDDeclinedBytesPair is unused (U1000)
tikv/raftstore/fsm_store.go:923:6: func calcRegionDeclinedBytes is unused (U1000)
tikv/raftstore/io_limiter.go:20:2: field limiter is unused (U1000)
tikv/raftstore/msg.go:55:2: const msgDefaultChanSize is unused (U1000)
tikv/raftstore/peer.go:466:50: should use time.Since instead of time.Now().Sub (S1012)
tikv/raftstore/peer.go:1072:7: should use !bytes.Equal(state.RequestCtx, read.binaryId()) instead (S1004)
tikv/raftstore/peer.go:1085:7: should use !bytes.Equal(state.RequestCtx, read.binaryId()) instead (S1004)
tikv/raftstore/peer.go:1467:21: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/peer.go:1489:20: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/peer.go:1493:20: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/peer.go:1530:23: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/peer.go:1591:23: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/peer.go:1705:44: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/peer_state.go:102:2: field closeCh is unused (U1000)
tikv/raftstore/peer_storage.go:203:6: func recoverFromApplyingState is unused (U1000)
tikv/raftstore/peer_storage.go:499:22: this value of err is never used (SA4006)
tikv/raftstore/raft_client.go:122:2: should write v := c.addrs[storeID] instead of v, _ := c.addrs[storeID] (S1005)
tikv/raftstore/snap.go:140:6: func copySnapshot is unused (U1000)
tikv/raftstore/snap.go:506:10: empty branch (SA9003)
tikv/raftstore/snap_test.go:48:72: undeclared name: DBBundle (compile)
tikv/raftstore/ticker.go:100:20: using time.Tick leaks the underlying ticker, consider using it only in endless functions, tests and the main package, and use time.NewTicker here (SA1015)
tikv/raftstore/ticker.go:107:18: should omit value from range; this loop is equivalent to for regionID := range ... (S1005)
tikv/raftstore/util.go:321:20: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/util.go:325:20: error strings should not end with punctuation or a newline (ST1005)
tikv/raftstore/worker.go:50:2: const taskTypeCompact is unused (U1000)
tikv/raftstore/worker.go:51:2: const taskTypeCheckAndCompact is unused (U1000)
tikv/raftstore/worker.go:154:6: type compactTask is unused (U1000)
tikv/raftstore/worker.go:158:6: type checkAndCompactTask is unused (U1000)
tikv/raftstore/worker.go:184:2: field closeCh is unused (U1000)
tikv/raftstore/worker.go:440:43: func (*pendingDeleteRanges).remove is unused (U1000)
tikv/raftstore/worker.go:450:43: func (*pendingDeleteRanges).timeoutRanges is unused (U1000)
tikv/raftstore/worker.go:482:24: func stalePeerInfo.timeout is unused (U1000)
tikv/raftstore/worker.go:626:74: should use time.Since instead of time.Now().Sub (S1012)
tikv/raftstore/worker.go:651:19: plainFileUsed is a pure function but its return value is ignored (SA4017)
tikv/raftstore/worker.go:876:24: func (*regionRunner).shutdown is unused (U1000)
tikv/region.go:98:22: func (*regionCtx).lessThanStartKey is unused (U1000)
tikv/region.go:106:22: func (*regionCtx).greaterThanEndKey is unused (U1000)
tikv/region.go:496:21: using time.Tick leaks the underlying ticker, consider using it only in endless functions, tests and the main package, and use time.NewTicker here (SA1015)
tikv/server.go:29:2: field wg is unused (U1000)
tikv/server.go:44:20: func (*Server).checkRequestSize is unused (U1000)
tikv/server.go:134:8: this value of err is never used (SA4006)
tikv/write.go:185:21: using time.Tick leaks the underlying ticker, consider using it only in endless functions, tests and the main package, and use time.NewTicker here (SA1015)
tikv/write.go:231:34: should use make(chan struct{}) instead (S1019)