Performance improvements

[Choumiko]

• especially for maintenance mode: keep track of areas (and positions?) that have already been prepared (trees/stone removed) and don't prepare again
• use count_entities_filtered for item-on-ground. Possibly: switch between using both and only find depending on whether an enttity was found in the last x calls (would reduce api calls when driving through big forests i think)
• rewrite maintenance mode; it's awful

[Choumiko]

Seems one big issue are long pieces of tracks without signals (for whatever reason): Simplest solution: place/remove fake signals every other tile? see: http://www.factorioforums.com/forum/viewtopic.php?f=48&t=8209