Foundational work for Future Memory usage improvements

This commit doesn't do much on its own, but adds a new Java Cleaner API
that lets us hook into Garbage Collector events to reclaim pooled objects and
return them to the pool.

Adds framework for Network Packets to know when a packet has finished dispatching
to get an idea when a packet is done sending to players.

Rewrites PooledObjects impl to properly respect max pool size and remove
almost all risk of contention.

Bumps the Paper Async Task Queue to use 2 threads, and properly shuts it down on shutdown.

Spigot-Server-Patches/Asynchronous-chunk-IO-and-loading.patch

@@ -1587,11 +1587,8 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +            // apply fixes
 +
 +            try {
-+                if (chunkData.poiData != null) {
-+                    chunkData.poiData = chunkData.poiData.clone(); // clone data for safety, file IO thread does not clone
-+                }
 +                chunkData.chunkData = chunkManager.getChunkData(this.world.getWorldProvider().getDimensionManager(),
-+                    chunkManager.getWorldPersistentDataSupplier(), chunkData.chunkData.clone(), chunkPos, this.world); // clone data for safety, file IO thread does not clone
++                    chunkManager.getWorldPersistentDataSupplier(), chunkData.chunkData, chunkPos, this.world); // clone data for safety, file IO thread does not clone
 +            } catch (final Throwable ex) {
 +                PaperFileIOThread.LOGGER.error("Could not apply datafixers for chunk task: " + this.toString(), ex);
 +                this.complete(ChunkLoadTask.createEmptyHolder());