fix(iroh, iroh-relay)!: Optimise the relay datagram path through the MagicSock (#3062)

## Description

This refactors how datagrams flow from the MagicSock (AsyncUdpSocket) to
relay server and back. It also vastly simplifies the actors involved in
communicating with a relay server.

- The `RelayActor` managed all connections to relay servers.
  - It starts a new `ActiveRelayActor` for each relay server needed.
  - The `ActiveRelayActor` will exit when unused.
    - Unless it is for the home relay, this one never exits.
  - Each `ActiveRelayActor` uses a relay `Client`.
- The relay `Client` is now a `Stream` and `Sink` directly connected to
the `TcpStream` connected to the relay server. This eliminates several
actors previously used here in the `Client` and `Conn`.
- Each `ActiveRelayActor` will try and maintain a connection with the
relay server.
- If connections fail, exponential backoff is used for reconnections.
- When `AsyncUdpSocket` needs to send datagrams:
  - It (now) puts them on a queue to the `RelayActor`.
- The `RelayActor` ensures the correct `ActiveRelayActor` is running and
forwards datagrams to it.
  - The `ActiveRelayActor` sends datagrams directly to the relay server.
- The relay receive path is now:
- Whenever `ActiveRelayActor` is connected it reads from the underlying
`TcpStream`.
- Received datagrams are placed on an mpsc channel that now bypasses the
`RelayActor` and goes straight to the `AsyncUpdSocket` interface.

Along the way many bugs are fixed.  Some of them:

- The relay datagrams send and receive queue now behave more correctly
when they are full. So the `AsyncUdpSocket` behaves better.
- Though there still is a bug with the send queue not waking up all the
tasks that might be waiting to send. This needs a followup: #3067.
- The `RelayActor` now avoids blocking. This means it can still react to
events when the datagrams queues are full and reconnect relay servers
etc as needed to unblock.
- The `ActiveRelayActor` also avoids blocking. Allowing it to react to
connection breakage and the need to reconnect at any time.
- The `ActiveRelayActor` now correctly handles connection errors and
retries with backoff.
- The `ActiveRleayActor` will no longer queue unsent datagrams forever,
but flush them every 400ms.
- This also stops the send queue into the `RelayActor` from completely
blocking.


## Breaking Changes

### iroh-relay

- `Conn` is no longer public.
- The `Client` is completely changed.  See module docs.

## Notes & open questions

- Potentially the relay `Conn` and `Client` don't need to be two
separate things now? Though Client is convenient as it only implements
one Sink interface, while Conn is also a Frame sink. This means on Conn
you often have to use very annoying syntax when calling things like
.flush() or .close() etc.
- Maybe a few items from the `ActiveRelayActor` can be moved back into
the relay `Client`, though that would probably require some gymnastics.
The current structure of `ActiveRelayActor` is fairly reasonable and
handles things correctly. Though it does have a lot of client knowledge
baked in. Being able to reason about the client as a stream + sink is
what enabled me to write the good `ActiveRelayActor` though, so I'm
fairly happy that this code makes sense as it is.

If all goes well this should:
Closes #3008 
Closes #2971 
Closes #2951

## 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.

---------

Co-authored-by: Friedel Ziegelmayer <[email protected]>

Author: flub