Remove deadlock risk in firing async events

The PluginManager incorrectly used synchronization on firing any event
that was marked as synchronous.

This synchronized did not even protect any concurrency risk as
handlers were already thread safe in terms of mutations during event
dispatch.

The way it was used, has commonly led to deadlocks on the server,
which results in a hard crash.

This change removes the synchronize and adds some protection around enable/disable

Spigot-Server-Patches/0357-Add-source-block-to-BlockPhysicsEvent.patch

@@ -1,22 +0,0 @@
-From d81b977104ba95086af880aff9f93669ea99537f Mon Sep 17 00:00:00 2001
-From: Sotr <i@omc.hk>
-Date: Thu, 23 Aug 2018 16:14:12 +0800
-Subject: [PATCH] Add source block to BlockPhysicsEvent
-
-
-diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
-index 04d0fa1d..64d75934 100644
---- a/src/main/java/net/minecraft/server/World.java
-+++ b/src/main/java/net/minecraft/server/World.java
-@@ -590,7 +590,7 @@ public abstract class World implements IBlockAccess {
-                 // CraftBukkit start
-                 CraftWorld world = ((WorldServer) this).getWorld();
-                 if (world != null && !((WorldServer)this).stopPhysicsEvent) { // Paper
--                    BlockPhysicsEvent event = new BlockPhysicsEvent(world.getBlockAt(blockposition.getX(), blockposition.getY(), blockposition.getZ()), CraftMagicNumbers.getId(block));
-+                    BlockPhysicsEvent event = new BlockPhysicsEvent(world.getBlockAt(blockposition.getX(), blockposition.getY(), blockposition.getZ()), CraftMagicNumbers.getId(block), blockposition1.getX(), blockposition1.getY(), blockposition1.getZ()); // Paper - add source block
-                     this.getServer().getPluginManager().callEvent(event);
- 
-                     if (event.isCancelled()) {
--- 
-2.18.0.windows.1
-