MB-9862: reuse memcached sockets in find_missing_revs

pooled_memcached_client:execute_with_socket logic was only treating
{ok, _} as successfull return. Causing find_missing_revs which returns
triple to not return sockets to pool.

This is now fixed by treating any return of tuple which starts with ok
as successful response.

Change-Id: If7c1640beee74b4b1d153c3b34a4ce2d72f5bf80
Reviewed-on: http://review.couchbase.org/31750
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>