Performance improvements when polling contacts

Under certain circumstances the polling of contacts can drastically reduce the performance of the system. Now onepoll.php is only called with contacts that can be polled. (before this change the check was done only in onepoll.php, so it was called unnecessarily often.

Additionally there is now a check for a stale onepoll.php and the fetching of the siteinfo is now cached. This increases the performance drastically.