OPENDJ-1116 Introduce abstraction for the changelog DB


Moved the lastGeneratedChangeNumber to the ChangeNumberIndexDB implementation because it is the responsibility of the DB to manage such field (think of it like relational databases's sequence numbers).


ReplicationServer.java:
Moved lastGeneratedChangeNumber + relevant code to DraftCNDbHandler.
Removed getNewChangeNumber(), replaced with TODO ChangeNumberIndexDB.getLastGeneratedChangeNumber()

DraftCNDbHandler.java
Moved lastGeneratedChangeNumber here and converted it to AtomicLong + moved relevant code here from ReplicationServer.
In getMonitorData(), extracted methods, createChangeNumberAttribute() and getChangeNumber().

ChangeNumberIndexDB.java:
Added nextChangeNumber() and getLastGeneratedChangeNumber().

ECLServerHandler.java:
Consequence of the changes to ReplicationServer.