Make MASpawnThread logic cancellable.

This commit changes how MASpawnThread is started. Instead of simply scheduling a task and letting it run whenever, we now explicitly tell it to stop when the arena ends. This matters, because if the scheduled task does not get cancelled, it will run at the scheduled time. MASpawnThread's run() method has an early return to stop whenever the arena isn't running, but with unlucky timing, it's possible to start a new arena session before the task runs again, meaning the same MASpawnThread keeps scheduling itself on the previous scheduling loop.

Instead of this whacky approach, we now specifically have MASpawnThread track its own BukkitTask. Upon arena start, we instantiate a new MASpawnThread and call its start() method, which in turn schedules itself and stores a reference to the associated BukkitTask. Upon arena end, we call the MASpawnThread's stop() method, which explicitly calls the BukkitTask's cancel() method, effectively stopping/unscheduling the task.

This fixes the (very) long-standing "double wave spawner" bug that we haven't been able to reproduce for so long. Hooray!

changelog.md

@@ -23,6 +23,7 @@ These changes will (most likely) be included in the next version.
 - The new command `/ma ready` (`/ma rdy` for short) can be used as an alternative to the iron block for readying up.  
 - Total experience is now correctly stored, reset, and restored on arena join/leave. This fixes a potential bug where total experience could increase in the arena, but levels and progress would still get reset at arena end.
 - The per-arena setting `keep-exp` returns. If enabled, any experience collected during an arena session is added as a reward on death or when the final wave is reached.
+- Waves will no longer intermittently progress at double frequency in some arena sessions. This long-standing bug where waves progress at "double speed" has finally been fixed.
 
 Thanks to:
 - Sait for adding the /ma ready command