Fix intermittent failure in DataChangeListenerTest

Fixed an intermittent failure in testDataChangedWithNoSender due to timing if a
DeadLetter is received due to the Monitor message being dropped as the
unit tests don't set up the termination monitor actor. The test expects
no DeadLetter for DataChangedReply so added chanhes to ignore
DeadLetters for other messages.

Change-Id: I39ddc888de7fbff9201ac9f785baa6fec7cb8f2c
Signed-off-by: tpantelis <tpanteli@brocade.com>