Refactor chunkserver to generate layers of snapshots

A chunk might have multiple snapshots when we remove the assumption
that snapshots are always exported and deleted afterwards.

Changes are:
1. Introduce CSSnapshots to manage snapshots;
2. `correctedSn' no longer used for CoW logic.
3. Two new fields are introduced to `ChunkRequest':
  - snapSn, the snapshot sequence number;
  - snaps, all snapshot sequence numbers.

TODO:
1. managing of `snaps' field in client side.
2. CRUD of `internal' snapshots.
3. storage consumption and statistics - take care of CoW files.
4. Recover & clone of `internal' snapshots.
5. caching of meta objects, and take care of memory footprint.
6. optimize deletion - no need to merge if intended to delete both file and snapshots.
7. Remove `correctedSn' from `ChunkFileMetaPage' and 'CSChunkInfo'.

UT:
1. `test/chunkserver/clone/clone_test' still fails.
2. new unit tests is still under development.

Signed-off-by: David Lee <live4thee@gmail.com>

proto/chunk.proto

@@ -57,12 +57,13 @@ message ChunkRequest {
     optional uint32 size = 7;           // for read/write/clone 读取数据大小/写入数据大小/创建快照请求中表示请求创建的chunk大小
     optional QosRequestParas deltaRho = 8; // for read/write
     optional uint64 sn = 9;             // for write/read snapshot 写请求中表示文件当前版本号，读快照请求中表示请求的chunk的版本号
-    optional uint64 correctedSn = 10;   // for CreateCloneChunk/DeleteChunkSnapshotOrCorrectedSn 用于修改chunk的correctedSn
+    optional uint64 snapSn = 10;        // for CreateCloneChunk/DeleteChunkSnapshot
     optional string location = 11;      // for CreateCloneChunk
     optional string cloneFileSource = 12;   // for write/read
     optional uint64 cloneFileOffset = 13;   // for write/read
     optional uint64 fileId = 18;  // for io fence
     optional uint64 epoch = 19;  // for io fence
+    repeated uint64 snaps = 20 [packed = true];
 };
 
 enum CHUNK_OP_STATUS {

proto/nameserver2.proto

@@ -72,6 +72,9 @@ message FileInfo {
     optional    uint64      stripeCount = 16;
 
     optional    uint64      epoch = 18;
+
+    // sequence numbers of snapshots
+    repeated    uint64      snaps = 19;
 }
 
 // status code

src/chunkserver/chunk_service.cpp

@@ -388,9 +388,9 @@ void ChunkServiceImpl::DeleteChunkSnapshotOrCorrectSn(
         return;
     }
 
-    if (false == request->has_correctedsn()) {
+    if (false == request->has_snapsn()) {
         response->set_status(CHUNK_OP_STATUS::CHUNK_OP_STATUS_INVALID_REQUEST);
-        LOG(ERROR) << "delete chunk snapshot failed, no corrected sn:"
+        LOG(ERROR) << "delete chunk snapshot failed, no snapshot sn:"
                    << request->logicpoolid() << "," << request->copysetid();
         return;
     }