Implement cancellation support/protection for module callbacks

[squahtx]

Update module callbacks with read semantics to protect them against
cancellation and avoid swallowing CancelledErrors.

Other module callbacks, such as the on_* callbacks, are presumed to
live on code paths that involve writes and aren't cancellation-friendly.
These module callbacks have been left alone.

Includes changes from #12468.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[squahtx]

Going to close this for now. A bunch of tests need updating first to not use the Mock().return_value = defer.succeed(...) pattern, since Deferreds aren't supposed to be awaitable more than once.