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/Chunk-debug-command.patch

@@ -226,7 +226,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +import java.util.Set;
  import java.util.concurrent.CompletableFuture;
  import java.util.concurrent.ExecutionException;
- import java.util.concurrent.Executor;
+ import java.util.concurrent.LinkedBlockingQueue;
 @@ -0,0 +0,0 @@ public final class MCUtil {
  
          return null;