OPENDJ-1116 Introduce abstraction for the changelog DB

Added ChangelogDB interface to abstract all the remaining code away from JE.



ChangelogDB.java, ChangelogState.java, Pair.java: ADDED

JEChangelogDB.java: ADDED
Created from code in ReplicationServer and ReplicationServerDomain.


ReplicationServer.java:
Moved a lot of code to JEChangelogDB.
Added ChangelogDB field and delegated existing method calls to it.
Added getChangelogDB().
Made getChangeNumberIndexDB() package protected for the tests.

ReplicationServerDomain.java:
Moved a lot of code to JEChangelogDB.
Replaced sourceDbHandlers field with ChangelogDB field.
Renamed publishMessage() to publishUpdateMsg().
FIXME!!!! Bug: generationIdSavedStatus is not protected by synchronized (sourceDbHandlers) !!!!!

ReplicationDbEnv.java:
Changed void initializeFromChangelogStateDB() to ChangelogState readChangelogState()


ExternalChangeLogTest.java:
Moved from package org.opends.server.replication to org.opends.server.replication.server.
Extracted method getReplicationDomainStartState()

ChangelogException.java:
Changed one ctor visibility to public.