glusterd: Disable transport before cleaning up rpc object

Problem:
rpc_transport object, which is part of rpc_clnt, is destroyed
prematurely. This is because, rpc_transport object is ref'd by socket
layer and rpc layer. These ref's, until the synctask'izing of
operations, were unref'd sequentially in the epoll thread.
With more threads at play, the sequential unref guarantee is off.

Fix:
Shutting down the transport before proceeding with cleaning up of
rpc_clnt object would serialize the unref's on the rpc_transport object
and thus eliminating the race.

Also, we don't store the address of brickinfo in brick's rpc notify
function, to avoid the possibility of referring a freed brickinfo.
Instead we use a string based id to 'reach' the corresponding brickinfo.

Change-Id: If2739e2eeaee1e8b071ab2b6754b7ea0f81cfceb
BUG: 962619
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/5000
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>