Make TCP_NODELAY tunable

Export MEMCACHED_DISABLE_TCP_NODELAY to disable it

Change-Id: Ie12be0151ddc51aeb5f355a72dceb27c947cd746
Reviewed-on: http://review.couchbase.org/25889
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>