CraftBukkit breaks legacy world conversion in three ways:
- Writes userdata to the path of the userdata folder rather than to
the correct file inside the aforementioned folder. This causes the
userdata folder to fail to be created as a file already exists at
its path.
- Makes changes to how multiworld works, without modifying
McRegionUpgrader to be aware of these changes.
- Calls methods on Bukkit before the server is initialized.
This patch fixes all of these issues, and also threads the
McRegionUpgrader to improve performance.
