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/0064-ProfileWhitelistVerifyEvent.patch

@@ -1,4 +1,4 @@
-From c814474d6c5d9c4d7142d141e6dabf3935a577cb Mon Sep 17 00:00:00 2001
+From 225e4e635f54b9f4ba440a4a849a4bddde5c6309 Mon Sep 17 00:00:00 2001
 From: Aikar <aikar@aikar.co>
 Date: Mon, 3 Jul 2017 18:11:34 -0500
 Subject: [PATCH] ProfileWhitelistVerifyEvent
@@ -124,5 +124,5 @@ index 00000000..59b69b23
 +    }
 +}
 -- 
-2.13.3
+2.14.1