mcserver: log if vbucket master is heuristically changed

Change-Id: Ic700abeccfe1ad0b68898c54bc7f524ea68caa36
Reviewed-on: http://review.couchbase.org/42402
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>