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/0100-Optimize-UserCache-Thread-Safe.patch

@@ -10,11 +10,11 @@ Additionally, move Saving of the User cache to be done async, incase
 the user never changed the default setting for Spigot's save on stop only.
 
 diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
-index a5aa6553cc5d44ad9be41a4f35de73a1c5bf3cb8..35d31a9c36e837fcc361e8ba82c89aaee319b73d 100644
+index d4434f10b879d9e2119b8adb6757b771e9df04f7..629304c403c596bf81dd8de919f0fcb5c77bd403 100644
 --- a/src/main/java/net/minecraft/server/MinecraftServer.java
 +++ b/src/main/java/net/minecraft/server/MinecraftServer.java
-@@ -760,7 +760,7 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
-         // Spigot start
+@@ -763,7 +763,7 @@ public abstract class MinecraftServer extends IAsyncTaskHandlerReentrant<TickTas
+         } catch (java.lang.InterruptedException ignored) {} // Paper
          if (org.spigotmc.SpigotConfig.saveUserCacheOnStopOnly) {
              LOGGER.info("Saving usercache.json");
 -            this.getUserCache().c();