MB-12238: set SO_KEEPALIVE on xmem and ssl proxy sockets

As another guard against possible issues when NAT forgets about
connections between datacenters.

Change-Id: Ia221bf5ff53fc564ccac1df8edcb02adf8f50084
Reviewed-on: http://review.couchbase.org/41589
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>