Warn if maxzoom in renderd.conf is set higher than the
 compile time specified variable in renderd

Currently renderd doesn't actually respect the maxzoom variable in renderd.conf. (only mod_tile does)
Changed the default supported max zoom of rendrd to 20

While mod_tile could handle much higher zoom levels already, renderd was stuck with z18,
unless one changed the compile time setting.

This changes the maximum supported zoom level to z20 out of the box. Furthermore,
it is now possible to set max and min zoom level per style in renderd.conf (as long
as they are within the range.)

Zoom levels higher than z20 are currently not supported out of the box, especially with
the file based storage backend. The directory hash function only supports 40 bit (20 bit for each X and Y).