Remove stages from zone schema

[Panzerhandschuh]

• Drop support for unordered stages (for now at least)
• Eliminate the "stages" section of ZoneDefs entirely, and make them inherently based on the main track
• Add a boolean setting to main track named something like stagesEndAtStageStarts which would determine whether stage end zones are generated from the next segment start or the last checkpoint of the current segment. This would be on by default and most maps will keep it on, but some jump maps will want it off even without a map being unordered