Kill Dynamic Actors when we're done with them

Kill ShardTransaction on close and on ThreePhaseCommitCohort#commit
Kill ThreePhaseCommitCohort on commit

Change-Id: Ie86b66cf3841baa514d82509fbc5b817eb7c6740
Signed-off-by: Moiz Raja <moraja@cisco.com>
Merge "Kill Dynamic Actors when we're done with them"