Ignore DataTreeChangeReply message

DataTreeChangeListenerActor is sending replies towards shard on each
processed datatree change. This is not handled by Shard, which ends up
flooding dead letters.

Make sure we ignore these messages, so that the logs are cleaner. Also
speed up the non-null check in DTCLActor.

JIRA: CONTROLLER-1986
Change-Id: I11b172d778c5b8cdf16aec58e77ec7faa9b6b146
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit d6bc529eaf624afe74d9bf9ee9ab0cb926bba576)