Expose transferList on MessageEvent #4521

I sometimes find myself using a Man-in-the-middle pattern with MessageChannel:

          +------------------------------+
          |                              |
portA1 =======> portA2   f()   portB1 =======> portB2
          |                              |
          +------------------------------+

Any message that gets sen via portA1 can eventually be received at portB2, but the function f has the opportunity to mangle, log or otherwise manipulate the message payload before it gets forwarded.

If the original message is sent with a transferList, the function has no way of accessing that transferList and forwarding the message with the same transfer behavior.

I think it would be good if the transferList could be exposed on MessageEvent.

Talking to @domenic OOB, this could be useful for the discussion around transferable values in the context of transferable Streams.