Show the actual seconds offset on the processlist page

[geoffreyanderson]

Sometimes when looking at the processlist page for a given raingauge sample, some of the seconds will be missing so there's not a full 30 pages of processlist to view. Since this implies that some time intervals simply don't have a collection, we should change it to reflect which seconds of the collection were retrieved.

E.g. Instead of "1->2->3->4->5->6", if the 3rd and 4th second of collections are missing, show "1->2->5->6" so that if a processlist snapshot is missing, it's harder to misdiagnose.

This change would probably need to be made in the get_file_pages() function in lib/RainGaugeModel.php.