Improve and simplify AsyncTask thread-local storage

This is now self-maintaining and doesn't rely on the async pool to wipe its ass on task completion. Instead, the garbage collector will cause thread-local data to be automatically released when the async task is garbage-collected on the main thread.

tests/plugins/TesterPlugin/src/pmmp/TesterPlugin/tests/AsyncTaskMemoryLeakTest.php

@@ -54,7 +54,7 @@ class TestAsyncTask extends AsyncTask{
 		usleep(50 * 1000); //1 server tick
 	}
 
-	public function __destruct(){
+	protected function reallyDestruct() : void{
 		self::$destroyed = true;
 	}
 }