Eliminate dead letters message when there's no sender

Eliminate dead letters message when there's no sender

When sending a message to close a DTCL or DCL registration, if the caller
isn't interested in the reply, it passes ActorRef.noSender() (ie null). Internally
akka translates this to the dead letters actor so you see log messages
of the form "Message [...CloseDataTreeChangeListenerRegistrationReply]
from Actor[...] to Actor[akka://opendaylight-cluster-data/deadLetters] was not
delivered. [1] dead letters encountered.".

To alleviate this we should check if the sender is the dead letters actor
before sending the reply.

Change-Id: Idfaf280acf720cf5727393262638a7783c1af539
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>