don't insist on unique name for ns_memcached_sup instances

Which breaks multiple buckets because every bucket creates its own
instance of ns_memcached_sup which all try to register under same name
and fail.

Change-Id: I7ee8b55ca6b7f6388f6276ccf8c3fe21e04fb69e
Reviewed-on: http://review.couchbase.org/42433
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>