Better operation cleanup

[obmarg]

#23 (and it's predecessor #19) added a stop_operation function that can be called to send a Complete message when a subscription is done. There's some improvements that could be made though:

1. Stopped operations will still be in the operation map. Should figure out a way to remove them.
2. If you drop a StreamingOperation it'll never be stopped. Ideally need a way to run stop_operation on drop (currently it's an async operation which makes this tricky - might need a way to do this that doesn't rely on async/await
3. Not quite related, but I'm noting it down here anyway: if the server stopped the subscription we ideally need a way to tell how & why on the StreamingOperation itself.

[Sytten]

I think we could also switch from using async mutex to parking_lot mutex since we don't keep the lock across await.
This means there is no need for and async mutex and it's much slower than a sync mutex.

[obmarg]

Yeah, up for that if you think it makes sense @Sytten