Support running events via CLI

If, for example, the plugin isn't loaded, this will reduce the number of unnecessary WP-CLI calls.

Default interval is fine for most sites, but high-volume sites may benefit from more-frequent checking. Doing so can also reduce worker idleness.

Improve errors for invalid paths, as they currently don't indicate what's wrong, just that something is wrong

Fixes #124

Functions that return a specific struct can't use `nil` though:

> cannot use nil as type siteInfo in return argument

Does change behaviour slightly, in that first iteration includes the delay.

Merge branch 'add/cli-orchestration' of github.com:Automattic/Cron-Control into add/cli-orchestration

Allow queue size and concurrency limits to be overridden

Only "normal" output should be heartbeats and things from init/main

Fixed sizes faired well to start, but it's now proven useful to be able to increase these limits in a controlled fashion.

Fixes #122

Free locks after timeouts and other uncatchable errors