OPENDJ-1496 (CR-3767) ThreadInterruptedException (JE) when running replication tests

Issue: On resetting the generationId in RS2, the ServerReader thread is blocked waiting on the ChangeNumberIndexer thread that is waiting on new changes.
The fix consisted in ensuring the replicaDB cursors are released by the ChangeNumberIndexer thread when a replicaDB is being cleared.


ChangeNumberIndexer.java:
Changed AtomicBoolean doClear field into ConcurrentSkipListSet<DN> domainsToClear.
Changed removeAllCursors() to removeCursors(DN baseDN).
In clear(), added a DN parameter.

JEChangelogDB.java, FileChangelogDB.java:
Called ChangeNumberIndexer.clear() from removeDomain() rather than from clearDB().