OPENDJ-1190 (CR-2523) Under rare circumstances the DS replication recovery thread (RSUpdater) can spin


This change is linked to the misbehaving RSUpdater thread which (wrongly):
- could be started multiple times
- would not shutdown willingly when the server is shutting down
- would try to look for replay operations in the future



LDAPReplicationDomain.java:
Added AtomicReference for RSUpdater thread.
Used DirectoryThread.isShutdownInitiated() for ServerStateFlush and RSUpdater threads.
In RSUpdater, added shutdown field to pass it down to buildAndPublishMissingChanges() + overrode initiateShutdown() to set the shutdown field to true.
In shutdown(), called initiateShutdown() for RSUpdater and RSUpdater threads.
In sessionInitiated(), only start the RSUpdaterThread if it does not already exist.
In buildAndPublishMissingChanges(), added early exits in case of shutdown.
Added now() method.

HistoricalCsnOrderingTest.java:
Consequence of the change to LDAPReplicationDomain.