Decrypt short URLs after N days

Here's a patch that could help users of the Drupal 6.x version of this module who are getting stuck with shorturl_link tables that grow to millions of rows, taking up gigabytes of valuable database diskspace.

The premise is that some time following the publication of a newsletter, say a month or so, keeping track of clicks becomes less important. However, one can't simply truncate the shorturl_link table as this would break the links in newsletters that have been sent out.

With this patch, we introduce a new setting which controls the decryption of simplenews_statistics short URLs into direct URLs, thus reducing required disk space while maintaining link integrity. The setting takes a number of days, beyond which URLs will become candidates for decryption. The process is triggered by cron and a maximum of 100000 records are processed on each cron run in order to avoid timing out when processsing a large shorturl_link table with millions of rows.