soldiers attack independently
currently, every 5 seconds, all units in a combat instance attach each other (
clash()). If we allowed units to attack independently, we could buff/debuff attack speed, which could add to players' strategies.
one possible issue: units with exactly the same speed. I don't want to randomly choose one over the other - combat should be fair (not favoring either team) and deterministic (random sorting accidents don't change the result). if speed is exactly the same, both should attack, and if both kill the other guy at the same time, draw. this is easy to implement when combat's one big function, but it's trickier (yet entirely possible) to make sure independent-attacks behave the same way.