In addition to the request queue and the dirty queue, there are now also a requestPrio and requestBulk queue.
The rendering order now is first render requests from requestPrio, then from request followed by the
dirty queue and finally if no other requests are queued, render from the requestBulk queue.
RequestPrio, Request and RequestBulk all block, whereas Diry immediately returns with NotDoneYet.
This also changes mod_tile to submit requests that if not rendered in time would result in 404 errors as high priority.
prioBulk should be useful for things like rerendering all the outdated tiles in the background, but this patch
does not include those changes.
By eliminating the linear look up of the dirtyQueue for every newly added rendering request, it should be possible
to significantly increase the length of the dirty rendering queue. At the moment the queue length is set to 1000 metatiles,
which at a typical rate of 2 - 3 metatiles a seconds is a buffer of only 5 - 10 minutes. With a much longer
queue, the excess daytime requests can be moved into the quieter night time hours, which could improve the overall long-term
throughput if the server is at its limit of what it can handle.
All operations on the dirty queue should now be O(1).
For large setups of mod_tile, and renderd, it may be beneficial to have a reverse proxy infront of the main tile server.
However when using a fast updating db e.g. running off the minutly diffs, the proxy may either deliver stale content, or the
cache expiry has to be set very short, which limits the effectiveness of the proxy. Instead, this patch adds an explicit
mechanism to renderd to signal to the proxy which tiles have been rerendered and thus should be discarded from the cache.
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
