Work around non-blocking fd issue on Windows.

Summary:
I reported the Lwt issue here:
ocsigen/lwt#574. The long story short is that
reading from non-blocking fds on Windows doesn't seem to yield to other
threads, even when the fd isn't ready to read. I'm not sure the cause.

While we could just revert back to using blocking fds, we'd need to
either workaround ocsigen/lwt#569 or ask for
a lwt release and update lwt.

Reviewed By: ljw1004

Differential Revision: D7603653

fbshipit-source-id: 4af7e806e68cf188c16d199e98801be3b430ac54

Author: gabelevi

hack/utils/marshal_tools_lwt.ml

@@ -19,12 +19,14 @@ module Marshal_tools_lwt = Marshal_tools.MarshalToolsFunctor (struct
   let write ?timeout fd ~buffer ~offset ~size =
     if timeout <> None
     then raise (Invalid_argument "Use Lwt timeouts directly");
-    Lwt_unix.write fd buffer offset size
+    Lwt_unix.wait_write fd
+    >>= (fun () -> Lwt_unix.write fd buffer offset size)
 
   let read ?timeout fd ~buffer ~offset ~size =
     if timeout <> None
     then raise (Invalid_argument "Use lwt timeouts directly");
-    Lwt_unix.read fd buffer offset size
+    Lwt_unix.wait_read fd
+    >>= (fun () -> Lwt_unix.read fd buffer offset size)
 
   let log str = Lwt_log_core.ign_error str
 end)