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

Move lock functionality to its own class so it can be reused in CPT and other places that need rate-limiting

Values aren't changed anywhere, but are referrenced in multiple places.

See also 5791003

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

[skip ci]

[skip ci]

Each class doesn't need its own corresponding functions file, there just won't be enough functions to justify that

Removing class properties in favor of namespaced constants, since these things aren't variable or class-specific

Also fixes the abstract class to support multiple classes. 🤦

See a2c0141d8

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.