OPENDJ-1116 Introduce abstraction for the changelog DB

Renamed:
- ReplicationIterator to ReplicationDBCursor
- ReplicationIteratorComparator to ReplicationDBCursorComparator
- JEReplicationIterator to JEReplicationDBCursor

Fixed javadocs, variable names, method names, etc.

ReplicationServerDomain.java:
Renamed getChangelogIterator() to getCursorFrom().

DbHandler.java:
Renamed generateIterator() to generateCursorFrom().

MessageHandler.java:
Extracted nextOldestUpdateMsg() and findOldestChangeNumberFromReplicationDBs() to increase code expressiveness.