Follower: merge appending of entries and moving of commit-index

[belaban]

When a follower gets an AppendEntriesRequest, it appends the entries to its log (1 write to disk) and then tries moving commit-index (1 write to disk).
If we could merge these 2 operations into 1 (e.g. using the same WriteBatch in LevelDBLog), then we could double the performance of an AppendEntriesRequest at the followers.

[belaban]

Makes sense only LevelDBLog, not in FileBasedLog (latter has separate mem-mapped files for log entries and metadata). Also, LevelDBLog might get phased out in the future anyway

[belaban]

Alternative: move LevelDBLog to jgroups-extras ("graveyard"), so people can still use it :-)

[belaban]

Perhaps tool to convert between FileBasedLog and LevelDBLog...