TransportReceiverT::receive should return an Option to indicate a dropped connection

[AArnott]

TransportReceiverT today declares this method:

async fn receive(&mut self) -> Result<ReceivedMessage, Self::Error>;

But this doesn't permit the implementation to indicate a dropped connection. Can we change it to the following:

async fn receive(&mut self) -> Result<Option<ReceivedMessage>, Self::Error>;

Then the caller should treat a None result the same way it would treat a closed transport.

This helps toward #907.

[niklasad1]

I think it would be better to replace TransportSender/TransportReceiver abstraction around AsyncRead/AsyncWrite to support other transports etc. The current abstraction is not great but does the job for now

[AArnott]

You've got me interested. How does the existing TransportReceiverT trait fall short? What would you replace it with?

[niklasad1]

It's opinionated, contain optional close messages and ideally we would simplify to just sending and receiving messages without any extra stuff.

I think it will work fine with the change you propose but if I would re-write I would do it differently.

[AArnott]

Interesting. I know that web sockets have a special close frame, but I never understood why. IMO all transports should be prepared for an unexpected close, which is what reading 0 bytes (or None) indicates. What value might a special close frame add?