Enforce update frequency settings ($checkPeriod) even when triggered by a cron event.

The WordPress cron implementation does not guarantee that a cron hook will only be run once, or run exactly on schedule. For example, it's possible for an event to be triggered two or three times in quick succession due to race conditions. If we check for updates each time our hook is run, we could end up sending redundant update requests that waste bandwidth and server resources.

Instead, make sure at least $checkPeriod hours have elapsed since the last check before checking again. Allow for a small fuzz-factor (currently 20 minutes) to account for the inherent inaccuracy of WP cron.