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.

buildSrc/src/main/kotlin/com/velocitypowered/script/SetManifestImplVersionPlugin.kt

@@ -45,7 +45,7 @@ class SetManifestImplVersionPlugin : Plugin<Project> {
                         velocityHumanVersion = "${project.version} (git-$currentShortRevision)"
                     }
                 } else {
-                    velocityHumanVersion = archiveVersion.get()
+                    velocityHumanVersion = project.version.toString()
                 }
                 attributes["Implementation-Version"] = velocityHumanVersion
             }