MB-9880: improve packetization of xmem batches

Due to details of naggle's algorithm and due to each request being
it's own send call we had sub-optimal packetization of batches of xmem
commands.

I.e. for short request (find_missing_revs), first request is sent in
own separate packet (due to naggle), then second/third delayed
(waiting either full segment or ack to first packet). For large
requests batch last (e.g. doc bodies as part of set_metas), naggle
could cause last write (which tail is usually not full segment) to be
delayed for ack.

We're setting tcp_nodelay on our memcached sockets (or ssl proxy
upstream side). And using delay_send option of erlang to make those
bunches of sends to be sent as single syscall.

Verified using wireshark packet dump to have intended effect for batch
of 3 short find_missing_revs requests.

Change-Id: I31df9821056fa620aa8e2c9a636c4f6b066bac14
Reviewed-on: http://review.couchbase.org/31801
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>