OPENDJ-1116 Introduce abstraction for the changelog DB

MultiDomainServerState.java
In update() removed the call to ServerState.duplicate() and let client code decide whether it needs to call it.
In toString(), use dSTringBuilder.
Code cleanup, removed use of "this" and useless parentheses.

ReplicationServer.java:
In getLastECLCookie(), made the code more efficient: called ReplicationServerDomain.getLatestServerState() only if the baseDN is not excluded.

ECLServerHandler.java:
In buildDomainContexts(), Call ServerState.duplicate() before calling MultiDomainServerState.update().