Restart XWiki only after several consecutive failed checks

A single failed HTTP check (after curl's own retries) restarts XWiki. A wiki that is still starting, or a short network problem, causes a restart, and a wiki that cannot start at all (for example a wrong database password) is restarted again after every maintenance period, forever.

Restart only after N consecutive failures, then wait for a cooldown, and stop restarting (and alert) after M restarts within a time window.