Make only one race run at a time

Done for smaller servers so the population isn't divided among too many
races.

Would be nice to turn this in to an option instead of a dodgy hack.
Fix big where no new race would start if people left before the race started

This method is just all-round better
Add option for running multiple simultaneous public races
Add settings comments
Add option for just running a single public race at once

This is useful for smaller servers to avoid diluting the population
between races.

Set settings.simultaneousPublicRaces = false for the new behaviour.