transport/socket: don't try to set TCP_DELAY on unix domain sockets

Change-Id: I290cd983bd0dff2e32e5ee90a12e888a3b31c6fd
BUG: 969461
Signed-off-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-on: http://review.gluster.org/5954
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>