resolve metadata overwritten issue in PUBSUB-106.

1) PendingAddOp should not handle responses after already submitted the callbacks.
2) Change #resolveConflicts in changing ensemble to use LedgerMetadata#resolveConflicts, which is the right way to resolve conflicts.

the issue of PUBSUB-106 could be reproduced by the test case, which simulates how it happened.

RB_ID=130958
enable flushWhenQueueEmpty and disable adaptive group commit to speed up the hedwig tests.

RB_ID=130481
 Fix stat exported for number of stubs
 The number of stubs was not being decremented if a replacement occurred. This fixes the problem.

RB_ID=132588
 Force garbage collection when disk usage exceeds a configurable threshold
 This change addresses the issue where the bookie goes into read only mode without issuing a garbage collection before doing this. If the disk usage exceeds a configurable threshold, garbage collection is forced to reclaim space to prevent the bookie from going into read only mode.

RB_ID=132910