Some improvements in hlog and hlog split

Some improvements in hlog and hlog split

1) Try to clean old hlog after each memstore flush to avoid unnecessary hlogs split in failover. Now hlogs cleaning only be run in rolling hlog writer.

2) Add a background hlog compaction thread to compaction the hlog: remove the hlog entries whose data have been flushed to hfile. The scenario is that in a share cluster, write requests of a table may very little and periodical, a lots of hlogs can not be cleaned for entries of this table in those hlogs.

3) Rely on the smallest of all biggest hfile's seqId of previous served regions to ignore some entries. Facebook have implemented this in HBASE-6508 and we backport it to hbase 0.94 in HBASE-9568.

4) Support running multiple hlog splitters on a single RS and on master(latter can boost split efficiency for tiny cluster)

5) Enable multiple splitters on 'big' hlog file by splitting(logically) hlog to slices(configurable size, eg hdfs trunk size 64M)
support concurrent multiple split tasks on a single hlog file slice

6) Do not cancel the timeout split task until one task reports it succeeds (avoids scenario where split for a hlog file fails due to no one task can succeed within the timeout period ), and and reschedule a same split task to reduce split time ( to avoid some straggler in hlog split)

7) Consider the hlog data locality when schedule the hlog split task. Schedule the hlog to a splitter which is near to hlog data.

8) Support multi hlog writers and switching to another hlog writer when long write latency to current hlog due to possible temporary network spike?

This is a draft which lists the improvements about hlog we try to implement in the near future. Comments and discussions are welcomed.