Minimize lock time in LeveldbTimelineStore.discardOldEntities

In current implementation, LeveldbTimelineStore.discardOldEntities holds a writeLock on deleteLock, which will block other put operation, which eventually block the execution of YARN jobs(e.g. TEZ). When there is lots of history jobs in timelinestore, the block time will be very long. In our observation, it block all the TEZ jobs for several hours or longer.

The possible solutions are:

• Optimize leveldb configuration, so a full scan won't take long time.
• Take a snapshot of leveldb, and scan the snapshot, so we only need to hold lock while getSnapshot. One question is that whether snapshot will take long time or not, cause I have no experience with leveldb.