Change UdpMultiplexer to throw a SocketException with SocketError.Shutdown when sending on an inactive multiplexer.  This to address a possible race condition where a listening thread is in the process of sending when its underlying client has been stopped.  Although the race should be found and stamped out, this problem is still a case that should be handled.
Change UdpMultiplexer to throw a SocketException with SocketError.Shutdown
when sending on an inactive multiplexer.  This to address a possible
race condition where a listening thread is in the process of sending
when its underlying client has been stopped.  Although the race
should be found and stamped out, this problem is still a case that
should be handled.

briandealwis/gt,briandealwis/gt