OPENDJ-1453 (CR-3667) Change time heart beat change numbers should be synced with updates

Added a test case for the change number computation + fixed the code to support it.
In ChangeNumberIndexer.run(), fixed a bug where shutdown forces an insert in the Change number index DB.

ChangeNumberIndexerTest.java:
Added emptyDBTwoDSsOneKilled test.
Fixed timestamps in a few tests.

ChangeNumberIndexer.java:
Initialized nextChangeForInsertDBCursor field to avoid NPE on startup.
Added getOldestLastAliveCSN(), mightMoveForwardMediumConsistencyPoint() + used it in tryNotify().
Changed canMoveForwardMediumConsistencyPoint() to canMoveForwardMediumConsistencyPoint(CSN).
Removed mediumConsistency field, unused now.
In run(), fixed a bug where shutdown forces an insert in the Change number index DB.
Updated javadocs.

MultiDomainServerState.java, MultiDomainServerStateTest.java:
Added getOldestCSNExcluding() and isReplicaExcluded().