OPENDJ-1116 Introduce abstraction for the changelog DB


Moved the cnIndexDB + related code to the ChangelogDB implementation because under its responsibility.


ReplicationServer.java:
Moved cnIndexDB + related code to JEChangelogDB.
Moved clearGenerationId() code to JEChangelogDB.removeDomain().

ReplicationServerDomain.java:
The code ReplicationServer.clearGenerationId().


ChangelogDB.java:
Temporarily added clearCNIndexDB until I can sort out the can of worms created by folding this method into other methods.
Renamed newChangeNumberIndexDB() to getChangeNumberIndexDB().

JEChangelogDB.java
Moved cnIndexDB + related code here from ReplicationServer.