Add another priority level (RenderLow) to accommodate rerenders after style changes

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.