Use Stack in the munin graph of queue_time
Fix sending back NotDone from renderd immediately if
 request doesn't fit into a rendering queue

RenderPrio, Render and RenderLow queues each only have a size of 32 entries. If these are full,
then requests overflow into the dirty queue. If this happens, renderd should send the NotDone
back immediately so that mod_tile doesn't wait for potentially a long time.

This however only worked for Render requests, but not RenderPrio or RenderLow. This patch fixes this.
Set min value of 0 for munin graph render time spent by
Correct axis label for renderd_queue_time munin graph
Correct derivation expression for renderd_zoom_time munin graph
Merge pull request #45 from tomhughes/munin

Fix munin plugin bugs
add capability to export specific bbox or zoom
Merge pull request #47 from woodpeck/master

add capability to export specific bbox or zoom
Merge branch 'master' of https://github.com/openstreetmap/mod_tile