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/POM-Changes.patch

@@ -56,6 +56,19 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
              <scope>compile</scope>
          </dependency>
 @@ -0,0 +0,0 @@
+             <version>7.3.1</version>
+             <scope>compile</scope>
+         </dependency>
++        <dependency>
++            <!-- wrapper to use either java 8 sun cleaner or java9+ official cleaner -->
++            <groupId>co.aikar</groupId>
++            <artifactId>cleaner</artifactId>
++            <version>1.0-SNAPSHOT</version>
++        </dependency>
+         <!-- deprecated API depend -->
+         <dependency>
+             <groupId>com.googlecode.json-simple</groupId>
+@@ -0,0 +0,0 @@
  
      <!-- This builds a completely 'ready to start' jar with all dependencies inside -->
      <build>