MB-100 use sockets pool for long blocking memcached calls

so far it is just wait_for_checkpoint_persistence but there
will be more of them

Change-Id: I712f30a33fde34b1ba4216ef722de9ede4de49df
Reviewed-on: http://review.couchbase.org/32748
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>