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/0501-Implement-Mob-Goal-API.patch

@@ -5,10 +5,10 @@ Subject: [PATCH] Implement Mob Goal API
 
 
 diff --git a/pom.xml b/pom.xml
-index bc8438ae1ae939be4d2257d37df9a843a13ac301..0c0051f7f2a58d63b851e80b442acbb47ca75dfa 100644
+index 60bf6dd28527f6ffcb59779ec9546030245724ca..1ae57e8ab2681e674f97ef3220c5cbf9143c6a7e 100644
 --- a/pom.xml
 +++ b/pom.xml
-@@ -122,6 +122,13 @@
+@@ -128,6 +128,13 @@
              <version>1.3</version>
              <scope>test</scope>
          </dependency>