refactor(iroh-net): replace flume in iroh-net with async_channel (#2539)

## 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

- [x] Self-review.
- [x] ~~Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.~~
- [x] ~~Tests if relevant.~~
- [x] ~~All breaking changes documented.~~