BUG-7033: Fix commit exception due to pipe-lining

BUG-7033: Fix commit exception due to pipe-lining

The DistributedDataStoreRemotingIntegrationTest#testTransactionWithIsolatedLeader
has failed sporadically on commit with the "Store tree X and candidate
base Y differ" error due to an edge case bug with the pipe-lining. Basically
this occurs if tx 1 is pending completion of replication and tx 2 is progressed
to the COMMIT_PENDING state but the associated DataTreeCandidate has
ModificationType.UNMODIFIED. In that case we elide replication and proceed
immdiately to finishCommit which results in the error due to tx 2 committing
before tx 1. To fix it, I added a new FINISH_COMMIT_PENDING state and call
payloadReplicationComplete, which checks the head of the pendingFinishCommits
queue, when replication is elided..

Change-Id: I5a0d033df131c9c3f4e24670a02a971dec331a4d
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>