In-memory backend

[code-of-kpp]

Original idea was to create alternative to asyncio.Queue that preserves tasks after program restart and supports acks().

Now API is too different but sometimes it would be still a good idea to have an alternative to aiothreading and ack() support in asyncio.Queue-like object.

We can implement the same API we have but with local object without communication with outside world: just a wrapper around queue.Queue (for thread safety) or similar.

This would be also very useful for tests.