[mb_master] Set master to undefined if node's removed from peers.
[mb_master] Set master to undefined if node's removed from peers.

Again, that only happens for short period of time, but it's good to
have consistent state at all times.

Change-Id: Id483e11adcced20e62030740f1c7ab50c6755572
Reviewed-on: http://review.couchbase.org/86331
Tested-by: Aliaksey Artamonau <ce14b1c16e0fd654f926118eb8addbec5aff4ffa@couchbase.com>
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server