allow not to set bucket when taking socket from memcached pool

Change-Id: Iba1ad3588583a679a8cd6866e2eda65b9faf3eb1
Reviewed-on: http://review.couchbase.org/38380
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>