OPENDJ-1611 Check serverState before returning a message

Two updates to the fix:
- always checks serverState before returning a message in 
 MessageHandler.getNextMessage() method
- doesn't exclude source handler from connectedDSs in
 ReplicationServerDomain.take() method

git-svn-id: https://svn.forgerock.org/opendj/trunk/opends@11372 41b1ffd8-f28e-4786-ab96-9950f0a78031