significantly increase the load thresholds after which mod_tile reduces rendering

sets the new MaxLoadMissing to 50 and MaxLoadOld to 16.

With the prevalence of multi core servers today, the previous
default thresholds of 10 and 5 were becoming to lower than the average
CPU count. Furthermore, load is not an ideal measure, as it includes I/O
load and much of the I/O load might well come from serving rather than
rendering. Especially if the database is on a separate SSD. Lastly, using
the thresholds in mod_tile isn't super helpful, as due to hysteresis of
 rendering queues stop submitting tiles at a time of high load is not
particularly helpful without stopping rendering from the queue.