Chunkloading: Fix ticket releasing, add mod-based tickets, clear on start, etc

- The ticket releasing stuff on world load was completely messed up, I had
  misunderstood how that stuff works. Should be fixed now...
- Clear all the timeout and ticket stuff on server start, otherwise we end
  up using old references to worlds, and that causes nasty crap and NPEs.
- Change player data storage in tickets to use the NBTHelperPlayer stuff.
- Added mod-based (=non-player based) chunkloading.
- Before requesting a new ticket and forcing a chunk, check if we already
  have a timeout (and thus forced chunk loading) on the target chunk, and
  then just increase/reset the timeout if the new one is longer than the
  existing one.