export pooled_memcached_client:send_batch and extract_recv_socket

They implement {batch_socket, _} abstraction allowing external code to
work with such sockets.

Change-Id: I38a2cf4c54c379a7d431eff4b9b32db30cebc768
Reviewed-on: http://review.couchbase.org/37628
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>