Added close_backend_delay

This allows the proxy to wait some fixed amount of time after the client
unbinds the connection, before closing the backend sockets.  The reason
this is helpful is that sometimes, the :relay server happily responds
and the client closes the connection, but the :duplex server(s) have not
all responsed.  In this case, we see a ECONNRESET error on the servers,
which is inconvienent.