With unique constraints, this will help with situations where events with specific timestamps are changed

Looped until circuit breaker tripped because `$page` wasn't respected. Also revealed that an additional query was run when we knew it wasn't needed, because we got back fewer entries than we asked for.

When events are cleared with `wp_clear_scheduled_hook()`, they can't be rescheduled until the cleanup job runs.

Fixes #91

Fixes #90

CLI may use this eventually

`get_event()` method was removed earlier

Considerable cleanup and simplification along the way

This logic became spread across various classes during initial development, and it's time to consolidate.

When running events, lookups will be simplified with this already in the DB

Core resets the option before its tests, so we perform the equivalent. Addressing #90 will make the non-Core approach unnecessary.