While I was going through uses of preserve_fn for other PRs, I converted places
which only use the wrapped function once to use run_in_background, to avoid
creating the function object.
Twisted core doesn't have a general purpose one, so we need to write one
ourselves.
Features:
- All writing happens in background thread
- Supports both push and pull producers
- Push producers get paused if the consumer falls behind
Richard van der Hoff