CONTROLLER-1641: Integrate DataTreeCohortActor with pipe-lining

CONTROLLER-1641: Integrate DataTreeCohortActor with pipe-lining

The DataTreeCohortActor was originally written assuming that only
one 3-phase commit was in progress at any time and thus maintained
a single state. However with transaction pipe-lining there can be
multiple simultaneous 3-phase commits so DataTreeCohortActor was
modified to maintain/track state per transaction.

In addition, it now also handles the DOMDataTreeCommitCohort
Futures async.

Change-Id: Ib7588ea2e32b297a2db0b532726549f9ec61a1a4
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>