Module `Hvsock_lwt.Flow_shutdown`

Adds a layer of signalling over Hyper-V sockets to simulate unidirectional shutdown. This implements the same protocol as:

https://github.com/rneugeba/virtsock/tree/master/go/hvsock

module Make
  (Time : Mirage_time.S)
  (Fn : S.FN)
  (Socket_family : Hvsock.Af_common.S) : 
  sig ... end

A Mirage FLOW over a hypervisor socket with an additional protocol layer to workaround bugs where in-flight data is lost after a shutdown_write or a close.