refactor(iroh-net): replace flume in iroh-net with async_channel

[rklaehn]

Description

refactor(iroh-net): replace flume in iroh-net with async_channel

Rationale: see #2536

This is the first in a series of PRs that will replace flume with async_channel. In this case it is very close to a drop in replacement without any need for changes.

Only noteable changes:

send_async becomes send
send becomes send_blocking
Receiver implements Stream, so no need for .into_stream()
Receiver is not Unpin, so to make it unpin you need to do Box::pin(recv) or use .boxed()

Breaking Changes

None

Notes & open questions

• Anything specific from flume we were relying on here?

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.

[flub]

the problem with a pr like this is that you see all the crimes that are being committed with channels. but the change is basically equivalent so fine, just close my eyes.