OPENDJ-1116 Introduce abstraction for the changelog DB


Changes after review from Matt.


ChangelogDB.java:
Added removeDB().
Renamed clearDomain() to removeDomain().
In getCursorFrom(), never return a null Cursor anymore.
Updated many javadocs.



MessageHandler.java, ReplicationBackend.java:
Removed null checks for cursor (consequence of the changes to ChangelogDB)

ReplicationServerDomain.java, JEChangelogDB.java, JEReplicaDBCursor.java:
Consequence of the changes to ChangelogDB.


ReplicationServer.java:
Added removeDb().
Removed getDbDirName(), superseded in usage by removeDb().

ReplicationTestCase.java, ReplicationServerTest.java:
Consequence of the change to ReplicationServer.


DbHandler.java, ReplicationDB.java:
Renamed setCounterWindowSize() to setCounterRecordWindowSize() + updated javadoc.

DbHandlerTest.java:
Improved the tests.
Added testGetCountNoCounterRecords() to assess code behaviour.
Extracted method assertCount()

CSNGenerator.java:
Fixed ugly parameter name.