Finally migrating our Controller class from Illia's AsyncClassWrapper to our
Synchronous mixin.

Benefits are...

  * Class no longer requires a synchronous and asynchronous copy.

  * Controller can be implemented as a fully asynchronous class, while still
    functioning in synchronous contexts.

Downside is...

  * Python type checkers (like mypy) only recognice our Controller as an
    asynchronous class, producing false positives for synchronous users.