[Decorators] Add maybe_coroutine decorator

- Clean up callback hell by making more code inline
- Use async/await syntax as it has more modern niceties than inlineCallbacks
  - Also gets us closer if we want to transition to asyncio in the future
  - `await` is usable in places that `yield` is not. e.g. `return await thething` `func(await thething, 'otherparam')`
  - IDEs know async semantics of async/await syntax to help more than with `inlineCallbacks`
- `maybe_coroutine` decorator has nice property (over `ensureDeferred`) that when used in a chain of other coroutines, they won't be wrapped in deferreds on each level, and so traceback will show each `await` call leading to the error.
- All async functions wrapped in `maybe_coroutine` are 100% backwards compatible with a regular Deferred returning function. Whether called from a coroutine or not.
- Use Deferred type hints as strings since older versions of twisted
(<21.7) don't support generic Deferred type hinting.

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