Achieving a high cache hit ratio is both beneficial to clients and the server. For the clients it is
reduced loading time and for the server reduced resources used. To achieve this, cache expiries should be set
as long as possible. However, this conflicts with wanting up to date tiles, especially with minutely renderings.
As you can't predict the future, one can only use a bunch of heuristics.
With this patch, the tile expiry heuristic is based on three factors. The frequency of planet updates (the heuristic used so far),
the zoom level and the last modified time. The idea behind the zoom level heuristic is that low zoom tiles don't change noticably
very often, and so can use longer cache expiries than high zoom tiles. The last modified heuristic is based on the assumption
that a tile that hasn't changed for a while is more likely to stay unchanged than one that has recently been modified.
The heuristics have a bunch of tunables that can be set in the apache config of mod_tile
* -t|--tile-dir option for render_list and convert_meta
* renderd reads it from renderd.conf section "[renderd]" setting "tile_dir"
* mod_tile reads from Apache conf ModTileTileDir directive
There are still some places in store.c left, where HASH_DIR is used directly. More fixes needed.
Made renderd more configurable at runtime: socketname, num_threads and the mapnik settings plugins_dir, font_dir, and font_dir_recurse are now available through renderd.conf
There should be only one incompatible change: the [Default] section in the config is now in all lowercase "[default]" because the parser lib gives section and key names back in lowercase
