Handle 3PC message backwards compatibility

Handle 3PC message backwards compatibility

Modified the ThreePhaseCommitCohortProxy to send the appropriate 3PC
messages based on the backend cohort actor's version. The version is
supplied via a new CohortInfo class which also holds the existing cohort
ActorSelection Future. The caller actually specifies a Supplier to
obtain the version b/c the version may not be known until the
ActorSelection Future is known. The TransactionProxy passes a Supplier
which obtains the version from the TransactionContext. As a result, the
ThreePhaseCommitCohortProxy was refactored a bit to handle the CohortInfo
instances.

The ThreePhaseCommitCohortProxyTest was refactored to use real actors for
the cohorts instead of mocking messages via a mocked ActorContext.

Modified the ShardCommitCoordinator to return the appropriate versioned
replies.

Change-Id: I13e6acf81c1fe8abda43c30b1a73648e411ab500
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>