Make TCP_NODELAY tunable

Export MEMCACHED_DISABLE_TCP_NODELAY to disable it

Change-Id: I9a79e6eb1f40ae0c19c5d9b47760625e8ad0c745
Reviewed-on: http://review.couchbase.org/25882
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>