Disable Sync Events firing Async errors during shutdown

This is how it use to behave on Paper, and this is totally destroying
the ability to try to shut the server down gracefully during the
shutdown process as events firing on the watchdog thread are throwing
errors.

This isn't an issue on Spigot

This has caused me so many rollbacks on watchdog already :(

Spigot-API-Patches/Disable-Sync-Events-firing-Async-errors-during-shutd.patch

@@ -0,0 +1,26 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sat, 11 Apr 2020 21:38:59 -0400
+Subject: [PATCH] Disable Sync Events firing Async errors during shutdown
+
+This is how it use to behave on Paper, and this is totally destroying
+the ability to try to shut the server down gracefully during the
+shutdown process as events firing on the watchdog thread are throwing
+errors.
+
+This isn't an issue on Spigot
+
+diff --git a/src/main/java/org/bukkit/plugin/SimplePluginManager.java b/src/main/java/org/bukkit/plugin/SimplePluginManager.java
+index 8355f9f0e..4931bc86d 100644
+--- a/src/main/java/org/bukkit/plugin/SimplePluginManager.java
++++ b/src/main/java/org/bukkit/plugin/SimplePluginManager.java
+@@ -0,0 +0,0 @@ public final class SimplePluginManager implements PluginManager {
+         // Paper - replace callEvent by merging to below method
+         if (event.isAsynchronous() && server.isPrimaryThread()) {
+             throw new IllegalStateException(event.getEventName() + " may only be triggered asynchronously.");
+-        } else if (!event.isAsynchronous() && !server.isPrimaryThread()) {
++        } else if (!event.isAsynchronous() && !server.isPrimaryThread() && !server.isStopping() ) {
+             throw new IllegalStateException(event.getEventName() + " may only be triggered synchronously.");
+         }
+ 
+--