HADOOP-15880. Reduce redundant end logsegment rpc

[lamberken]

ISSUE

https://issues.apache.org/jira/browse/HDFS-15880

OUTLINE

The SNN always send end logsegment rpc in cycle（dfs.ha.log-roll.period=120s）.

• If no edit request occurs during this period, ANN will still send the RPC request to the journal node which is redundant.
• The FSEditLog#endCurrentLogSegment method is marked synchronized, it will affetc other rpc requests.

Editlogs only contains two operations :

LogSegmentOp [opCode=OP_START_LOG_SEGMENT, txid=-12345]
LogSegmentOp [opCode=OP_END_LOG_SEGMENT, txid=-12345]

[dcadmin@dw-work-006 ~]$ ll /work/data/hadoop/hdfs/journalnode/nn-work/current/
total 1100
-rw-r----- 1 dcadmin dcadmin       8 Mar  7 01:02 committed-txid
-rw-r----- 1 dcadmin dcadmin      42 Mar  7 00:35 edits_0000000000000000001-0000000000000000002
-rw-r----- 1 dcadmin dcadmin      42 Mar  7 00:37 edits_0000000000000000003-0000000000000000004
-rw-r----- 1 dcadmin dcadmin      42 Mar  7 00:39 edits_0000000000000000005-0000000000000000006
-rw-r----- 1 dcadmin dcadmin      42 Mar  7 00:41 edits_0000000000000000007-0000000000000000008
-rw-r----- 1 dcadmin dcadmin      42 Mar  7 00:43 edits_0000000000000000009-0000000000000000010
-rw-r----- 1 dcadmin dcadmin      42 Mar  7 00:45 edits_0000000000000000011-0000000000000000012
-rw-r----- 1 dcadmin dcadmin      42 Mar  7 00:48 edits_0000000000000000013-0000000000000000014
-rw-r----- 1 dcadmin dcadmin      42 Mar  7 00:50 edits_0000000000000000015-0000000000000000016