Prevent classloader leak in metadata system. Fixes BUKKIT-3854

Metadata values keep strong reference to plugins and they are not
cleared out when plugins are unloaded. This system adds weak reference
logic to allow these values to fall out of scope. In addition we get
some operations turning to O(1) "for free."

By: crast <contact@jamescrasta.com>

paper-api/src/main/java/org/bukkit/metadata/MetadataValueAdapter.java

@@ -1,5 +1,7 @@
 package org.bukkit.metadata;
 
+import java.lang.ref.WeakReference;
+
 import org.apache.commons.lang.Validate;
 import org.bukkit.plugin.Plugin;
 import org.bukkit.util.NumberConversions;
@@ -13,15 +15,15 @@ import org.bukkit.util.NumberConversions;
  *
  */
 public abstract class MetadataValueAdapter implements MetadataValue {
-    protected final Plugin owningPlugin;
+    protected final WeakReference<Plugin> owningPlugin;
 
     protected MetadataValueAdapter(Plugin owningPlugin) {
         Validate.notNull(owningPlugin, "owningPlugin cannot be null");
-        this.owningPlugin = owningPlugin;
+        this.owningPlugin = new WeakReference<Plugin>(owningPlugin);
     }
 
     public Plugin getOwningPlugin() {
-        return owningPlugin;
+        return owningPlugin.get();
     }
 
     public int asInt() {