CONTROLLER-1641: Handle commit cohorts async

CONTROLLER-1641: Handle commit cohorts async

Modified CompositeDataTreeCohort to handle response messages from
the DataTreeCohortActor async. The canCommit, preCommit, and commit
methods now return an Optional CompletionStage that is present when
there are commit cohorts. On completion of the cohort actor Futures, a
Runnable is executed in the shard actor to process the responses and
complete the returned CompletionStage. The call sites in
SimpleShardDataTreeCohort execute their own callbacks on completion.

Change-Id: Ib1a22e059dabfe87a1ee141baae655601f9ce700
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>