Explore alternative solutions for channel extraction multithreading

Boiled down version of @hughes036 and my discussion on `prediction/service.py` implementation of channel extraction:
- current solution works, but may not be idiomatic
- creating threads and destroying them may be too expensive
- constraints:
1. most recently created thread must be the only thread that has an effect on UI
2. cannot drop references to running threads (causes Qt to crash)
3. thread work should not 'pile up' (they should be able to run in parallel, as much as python is able to do that)
- things to look into:
1. `asyncio`
2. thread pool solutions (`QThreadPool` e.g.)
3. 'debouncing'