mux: ocaml: don't send Shutdown after Close
mux: ocaml: don't send Shutdown after Close

A badly-behaved client could send Shutdown after Close, for example
if the Proxy.proxy function in `vpnkit` throws an exception for one
of the directions, the function will return and the `Lwt.finalize`
will call `close`. Some time later the other copy will finish and
call `close_write` which will send a Shutdown message.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit