Proper state management

[kixelated]

Easily the most difficult part of this project has been how to handle state cleanup. Rust futures are not guaranteed to execute, so the code has to be littered with Drop traits everywhere to ensure we could cleanup at any point. This get extremely difficult when combined with the borrow checker, as often a child needs a reference to the parent to clean up a cache entry or whatever.

I spent a while but I think I've got something pretty solid. The core is the State helper that is like a fancy RWMutex. It's split into two halves that can be independently dropped, allowing us to track when the producer and all consumers are dropped. You can read at any time, but you can only wait or lock mutably when it's not dropped, preventing tasks from running when the other side is gone.

I added back the multi-origin support and it should now correctly handle things like deduplication and removing announces. Who knows though the code is really hard to reason about.