OPENDJ-1439 Change number stops progressing with cross domain replication 

The changeNumber progression was blocked when the mediumConsistencyCSN was from a different baseDN than the new change to add to the changeNumber index DB.
Fixed this problem by also storing the baseDN of the mediumConsistencyCSN in the ChangeNumberIndexer class.


ChangeNumberIndexer.java:
Renamed mediumConsistencyCSN to mediumConsistency and changed its type from CSN to Pair<DN, CSN>.
In tryNotify(), removed the now useless baseDN parameter (replaced with mediumConsistency field).
Improved comments.

ChangeNumberIndexerTest.java:
Renamed BASE_DN to BASE_DN1.
Added BASE_DN2 and test emptyDBTwoInitialDSsDifferentDomains().
In assertExternalChangelogContent(), did some renaming.

SequentialDBCursor.java:
Improved toString().

ChangeNumberIndexDB.java:
Removed obsolete FIXME.