Fix FindBugs warnings in sal-distributed-datastore and enable enforcement

Fix FindBugs warnings in sal-distributed-datastore and enable enforcement

Several warnings were suppressed via annotation with justification provided.
Other warnings that were fixed:
 - remove redundant implements in several classes
 - "The referenced methods have names that differ only by capitalization"
   warnings. This is checked across all classes for consistency. The main
   offender was getTransactionID vs. getTransactionId. I changed all methods
   to getTransactionId and associated fields to transactionId.
 - unsynchronized access to a field where access is synchronized
   elsewhere (in DataTreeChangeListenerProxy and DatastoreContextIntrospector).
 - catching Exception instead of catching more specific exception types that
   are thrown from the try block.
 - unconfirmed casts - verify via Preconditions check to avoid warning
 - unnecessarily calling toString() on a String instance
 - synchronizing an AtomicInteger instance (in ThreePhaseCommitCohortProxy) -
   not an issue in this case but changed to synchronize a separate Object
   in lieu of supressing the warning.
 - unsynchronized to SimpleDateFormat which isn't thread-safe (in ShardStats).
 - potential null-pointer access of 'shard' in ShardStats - changed to pass
   'shard' to the ctor in lieu of setter.
 - calling String#getBytes w/o specifying encoding (in DataTreeModificationOutput).
 - privileged access to create ClassLoader in ActorSystemProviderImpl although not
   likely a SecurityManager would ever be present.

Change-Id: I0a87208f3f200fbe4f78e950c21419fbab154d94
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>