Checkpoint commit for OPENDJ-1206 : Create a new ReplicationBackend/ChangelogBackend 
 to support cn=changelog
CR-4083

[Note: real merge of all changelog.file package content and ChangelogBackend to be done
 in one shot in a future commit]

Implementation of core features of the changelog backend:
* Initialization and finalization of the backend
* Search of the changelog, in cookie mode (with a cookie control) and
  draft compat mode (by change number)

Code changes:
* ChangelogBackend.java: 
   - implementation of search(), hasSubordinates(),  numSubordinates(),
     and getEntryCount() methods 

* ReplicationServer.java:
   - new dependency on ChangelogBackend and ECLEnabledDomainPredicate
   - new constructor with ECLEnabledDomainPredicate argument
   - new methods enableExternalChangeLog() and shutdownExternalChangelog()
     as future replacement of enabledECL() and shutdownECL()
   - new method getDomainDNs(Set<DN>) for retrieval of domain DNs but an 
     excluded set of dns
   - new method validateServerState(MultiDomainServerState, Set<DN>) for 
     checking coherency of given state with the replication server

* ReplicationDomainDB.java: 
   - new method getCursorFrom(MultiDomainServerState, PositionStrategy, Set<DN>)
     that exclude a given set of domain DNs from the cursor obtained

* FileChangelogDB.java, JEChangelogDB.java:
   - implementation of new method 
     getCursorFrom(MultiDomainServerState, PositionStrategy, Set<DN>)

* ECLEnabledDomainPredicate.java, ECLMultiDomainDBCursor.java:
  - update visibility to public in order to use these classes in ChangelogBackend

* ChangelogBackedTestCase.java:
   - test of ChangelogBackend class, built from ExternalChangeLogTest.java,
     with lots of renaming, refactoring, cleaning compared to original class
   - majority of tests are disabled until the changelog backend is branched into
     code (as these tests require a running server)

* MonitorTest.java:
  - update creation of  ReplicationServer class to use a custom 
    ECLEnabledDomainPredicate
  
 * replication.properties:
   - add messages for changelog backend