When `wp_next_scheduled()` is called incorrectly, it can lead to an endless loop of event creation. Locking will, at least, slow it down.

See also 5791003

Core doesn't expect CPTs to be ready until after `init` fires, so neither can we.

Also fixes the abstract class to support multiple classes. 🤦

Still need to:
* deal with unscheduling events
* ensure all events are captured (Jetpack/VaultPress seem to be missing)

Redundant, I know, but there should be no mystery.