Checkpoint commit for OPENDJ-1206 : Create a new ReplicationBackend/ChangelogBackend 
 to support cn=changelog
CR-4036

Preparatory work :  change implementation of virtual attribute providers related to external
 changelog to be independant of ECL workflow

*  ReplicationServer class:
** remove duplication of virtual attribute providers creation (for registering
    and deregistering)
** create virtual attribute providers by injecting the replication server instance

* FirstChangeNumberVirtualAttributeProvider, LastChangeNumberVirtualAttributeProvider, 
  LastCookieVirtualProvider classes:
** inject ReplicationServer in constructor in order to use it in getValues() method
** remove ConfigurationChangeListener implementation because no config change 
 can happen

* ChangelogBaseDNVirtualAttributeProvider class:
** remove ConfigurationChangeListener implementation because no config change 
 can happen