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/0057-Shoulder-Entities-Release-API.patch

@@ -1,4 +1,4 @@
-From 749b81ac44634b7b41a61d72bf05086b872c39c6 Mon Sep 17 00:00:00 2001
+From 3a5f33ba756ff0587b88c7b6340ff6368131f51e Mon Sep 17 00:00:00 2001
 From: Aikar <aikar@aikar.co>
 Date: Sat, 17 Jun 2017 15:04:51 -0400
 Subject: [PATCH] Shoulder Entities Release API
@@ -34,5 +34,5 @@ index 518aa2a9..3939d4af 100644
       * Gets the entity currently perched on the left shoulder or null if no
       * entity.
 -- 
-2.13.3
+2.14.1