killAllLoops() only cancelled the next scheduled tick without waiting for
in-flight dimension ticks to finish. The main thread then ran saveAllChunks
concurrently with worker threads (level.tick -> processUnloads) mutating the
non-thread-safe ChunkMap.toDrop, corrupting the fastutil map and hanging the
save forever.
Add ServerManager.stopLoopsAndWaitIdle() that kills all loops and spins until
each loop isTicking()==false (30s watchdog), and call it first in
MinecraftServerDimMixin.stopServer before killAllLoops(), so dimension ticks
fully finish before the main thread starts saving.
xxdd001