Checkpoint commit for OPENDJ-1206 : Create a new ReplicationBackend/ChangelogBackend 
 to support cn=changelog
CR-4439

Implementation of an additional validation of provided cookie when searching changelog
in cookie mode. 

Check for each state of the provided cookie that the csn is not older than the oldest CSN
available in each domain. If this is not the case, then an error 
ERR_RESYNC_REQUIRED_TOO_OLD_DOMAIN_IN_PROVIDED_COOKIE is returned. 

This check was done previously in the ECLServerHandler class but was not ported when
writing ChangelogBackend.  It also means I'm re-adding some methods deleted by the 
previous big clean done by Jean-Noel. 

Changes:
* ReplicationServer.java : 
  - add the check to validateServerState() method
  - refactor validateServerState() method for better readability
* ReplicationServerDomain.java : 
  - re-add getOldestState() method
* ReplicationDomainDB. java : 
  - re-add getDomainOldestCSNs(DN) method
* FileChangelogDB.java, JEChangelogDB.java : 
  - re-add implementation of getDomainOldestCSNs(DN) method