Ensure 1st healthcheck runs in the future, not immediately before dispatch

[ianmjones]

Fixes a problem whereby as soon as the healthcheck cron event was scheduled, it ran straight away, even before the background process was dispatched.

This resulted in the cron event starting the background process dispatch chain and the dispatch() itself often being redundant.

Now the healthcheck cron is scheduled to start at the allotted cron interval in the future, continuing to reschedule itself at that interval from then on until the background process completes or is cancelled.

The filter that fires to set the cron interval now only fires once rather than potentially twice if the WP_Background_Process subclass has a cron_interval property added.

A get_cron_interval() method has been added that enables determining what the background process's healthcheck cron interval is.

[eriktorsner]

LGTM 👍