OPENDJ-1116 Introduce abstraction for the changelog DB


ReplicationServerDomain.java:
Renamed getStartState() to getOldestState().

ServerState.java:
In duplicateOnlyOlderThan(), changed parameter from CSN to long.


ReplicationServer.java:
Consequence of the change to ServerState and ReplicationServerDomain.
In getECLChangeNumberLimits(), brought the database empty code to the top of the method: to get something more readable.

ECLServerHandler.java:
Consequence of the change to ServerState and ReplicationServerDomain.
Changed a few names / comments.

JEChangeNumberIndexDB.java
Consequence of the change to ReplicationServerDomain.


ExternalChangeLogTest.java
Consequence of the change to ReplicationServerDomain.
Code cleanup:
- removed gblCSN instance field
- inlined sleep()
- used connection.process*() + inlined runModifyOperation() and runDeleteOperation().
- renamed getReplicationDomainStartState() to getDomainOldestState() + added better asserts