Make all mpsc channels use tokio::sync::mpsc

[teor2345]

Motivation

In PR #3244, we changed some mpsc channels to use tokio::sync::mpsc, because we needed a non-async receiver method.

But some channels were left on futures::channel::mpsc, because they have an automatic Stream impl.

Suggested Solution

• Replace futures::channel::mpsc with tokio::sync::mpsc
• Use ReceiverStream where needed
• Fix up the error handling and error types

[teor2345]

I'm not sure this cleanup is actually possible, due to API differences.