Fixed the duplicate attribute values returned by "dc=replicationchanges".


ReplicationBackend.java:
Removed useless setBaseDNs().
In exportRootChanges(), used Collections.singletonList() + Rearranged the code to make it more explicit and reduce duplication.
Renamed processContainer() to writeChangesAfterChangeNumber() + changed signature.
Renamed processChange() to writeChange() + changed signature + removed calling Operation.setInternalOperation().
Extracted methods extractChangeNumber(), canContinue(),  addAttribute(), createBackupManager(),  getBackendDir().