Fix spurious brick disconnects

Spurious disconnect were caused by a race condition inside
rpc_transport_ref()/rpc_transport_unref() that allowed the refcount
to drop to zero while the transport was still in use. The race
condition is made possible because of an uninitiaized mutex
produced when socket_server_event_handler() copies the transport

This is a backport of I34fe097a0ac21b0dbf58f5eed84880e3fd9814f2

BUG: 764655
Change-Id: Ib6a7c736f28ccc67d05be45629cddc18a642c11f
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/4908
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>