OPENDJ-1116 Introduce abstraction for the changelog DB

Changes after review from Matthew Swift.


ChangelogDB.java, JEChangelogDB.java:
Removed getDBDirectoryName(), not needed anymore.
In getDomainServerIds(), now return an unmodifiable set.
In getDomainOldestCSNs() and getDomainNewestCSNs(), now return a ServerState object.

ReplicationServerDomain.java:
Consequence of the change to ChangelogDB.

ReplicationServer.java
Consequence of the change to ChangelogDB.
Added config instance field + removed all the instance field that are duplicating its info + added getConfiguredRSAddresses() to replace toHostPorts() + adapted the code + moved some javadocs.