OPENDJ-1116 Introduce abstraction for the changelog DB


After talking Matthew's over the phone review, did a few changes described down below.

*.java:
Changed changeNumber from int to long throughout the replication code base.

ECLUpdateMsg.java:
In getBytes(), did not complete the change to long because it would require a protocol version change.


ReplicaDBCursor.java:
Now extends Comparable.

ReplicaDBCursorComparator.java: REMOVED
Code has been moved to JEReplicaDBCursor.compareTo().


ChangeNumberIndexDB.java:
Does not extend Runnable anymore (mistake from earlier refactorings).

DraftCNDbHandler.java:
Now extends Runnable.