Cache generated EventExecutors (fixes #786)

the first 'major' change in this PR is to cache the generated event
executrs from the ASM class, by doing this we only generate a single
class for every method that we need an executor for, thus reducing the
number of classes that are needed, especially in cases where plugins
re/unregister events all the time.

The second change is to modify the generated classloader map, generated
classloaders are not held against the plugin itself but the classloader
that the event is declared in, the implication here is that we cannot
drop generated classloaders when a plugin disable, and so we use a guava
weak-key'd hashmap, downfall here is that classes won't be GC'd until
guava drops the generated classloader, however the first change should
deal with most of the grunt.

Spigot-API-Patches/0062-ensureServerConversions-API.patch

@@ -1,4 +1,4 @@
-From d97890f63015395b9fcf7bc382686ddfe5f3e266 Mon Sep 17 00:00:00 2001
+From c6ffdd4bda7232c5e22d96bc0c5e6f6a9c995401 Mon Sep 17 00:00:00 2001
 From: Aikar <aikar@aikar.co>
 Date: Wed, 4 May 2016 23:55:48 -0400
 Subject: [PATCH] ensureServerConversions API
@@ -61,5 +61,5 @@ index 188ae6d7..6bb19b9d 100644
 +    // Paper end
  }
 -- 
-2.13.3
+2.14.1