transport/socket: fix connect/disconnect races

We might receive a connect request while a disconnect is still in
progress, requiring more states and (the return of) poller generation
numbers to avoid redundant pollers.  We might also get either kind of
request from within our own rpc_transport_notify upcall, so we have to
avoid locking and use the PLEASE_DIE state instead.

Change-Id: Icbaacf96c516b607a79ff62c90b74d42b241780f
BUG: 970194
Signed-off-by: Jeff Darcy <jdarcy@redhat.com>
Reviewed-on: http://review.gluster.org/5137
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>