In addition to Render and RenderPrio, add another priority level of RenderLow.
The idea is that if you rerender all tiles by touching planet-import-complete and
thereby likely overloading the server's rendering capacity you want to prioritize rerendering
of tiles that were explicitly marked dirty due to data changes over rerendering
due to planet-import-complete updates.
This is achieved by splitting rendering requests by if they are "old", or "very old".
As the tile expiry sets back the modification date many years, mod_tile splits
rendering requests based on how old the tiles are. If they are no more than a
certain threshold (currently set to one year) old, they are rendered with lower priority.
