refactor(iroh-bytes)!: remove flume dependency (#2622)

## Description

This is mostly a pretty mechanical replacement of flume with
async_channel in the rebd blob store.

One not so tiny change is that to support closing a read or write
transaction after some delay, and because async_channel does not have
recv_timeout, we switch the entire actor loop to run *async* but on a
dedicated thread of the main runtime.

That way we can use `tokio::select! {..., timeout}`

## Breaking Changes

FlumeProgressSender was removed

## Notes & open questions

## Change checklist

- [ ] Self-review.
- [ ] 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.
- [ ] Tests if relevant.
- [ ] All breaking changes documented.