Fighting causes lag

Somehow I managed to not find this bug report in a search, thus the duplicate. Is it possible to move attachments between bug reports? Bug #550210 contains a savegame that shows this problem in action.

Suggested a "fix" in my fixes branch. It dramatically reduces CPU load of fights, and ensures that the CPU load stays proportional to the number of fighting soldiers.

However, fights still require a significant amount of CPU, and perhaps worse, the CPU usage is quite erratic; it doesn't distribute well over time, there are often peaks of usage. I don't see an easy way to fix that, but fortunately it should only be a real problem in truly epic battles taking place in choke points.

(I suppose the biggest problem is that the user interface becomes choppy when the game logic requires more CPU. We had some pie-in-the-sky discussion on possible solutions based on multi-threading on IRC, but that kind of thing is probably very far away.)

Released in build16-rc1