OPENDJ-1259 (CR-3563) Make the Medium Consistency Point support replicas temporarily leaving the topology


Added more tests about how changeNumber computation is performed WRT replicas offline messages and RS restarts.


ChangeNumberIndexerTest.java:
Tested additional scenarios: emptyDBTwoDSsOneInitiallyOffline(), emptyDBTwoDSsOneInitiallyWithChangesThenOffline(), emptyDBTwoDSsOneInitiallyPersistedOfflineThenChanges().
In several tests, called assertExternalChangelogContent() after calling startCNIndexer().
Renamed setDBInitialRecords() to setCNIndexDBInitialRecords().
Added more javadocs.
Used @Mock + MockitoAnnotations.initMocks() instead of several calls to Mockito.mock().

ChangeNumberIndexer.java:
Changed the code to support the additional scenarios.
Renamed allInitialReplicasArePastOldestPossibleCSN() to allInitialReplicasAreOfflineOrAlive().
Changed removeCursor() into getCursor().