Reintroduce sync event execution to the Velocity event system

This required a not-insubstantial number of bug fixes, since the sync support had bit-rotted somewhat. This PR also corrects a number of bugs.

Finally. the per-plugin executor services are now used to execute all async event tasks.

api/src/main/java/com/velocitypowered/api/event/Subscribe.java

@@ -43,6 +43,6 @@ public @interface Subscribe {
    *
    * @return Requires async
    */
-  boolean async() default true;
+  boolean async() default false;
 
 }