Checkpoint commit for OPENDJ-1174 Transfer responsibility for populating the ChangeNumberIndexDB to ChangelogDB


Properly implemented the medium consistency point algorithm with heartbeats and replicas going offline.


ChangeNumberIndexer.java:
Added replicasOffline instance field.
Added methods replicaOffline() and removeCursor().
In moveForwardMediumConsistencyPoint(), updated the code to cater for offline replica.

ChangeNumberIndexerTest.java:
Added new tests for heartbeat and replica going offline.


MultiDomainServerState.java, ServerState.java:
Added removeCSN().

MultiDomainServerStateTest.java, ServerStateTest.java:
Added more tests.