Worker: Unstack tasks in a synchronized block

this prevents any tasks still left in the queue on shutdown getting pulled out by the worker when we're attempting to shut it down.
This led to various race conditions, most notably weird cases where PopulationTask would inexplicably find its expected generator state had not been correctly set up.

src/thread/Worker.php

@@ -56,7 +56,9 @@ abstract class Worker extends \Worker{
 		$this->isKilled = true;
 
 		if(!$this->isShutdown()){
-			while($this->unstack() !== null);
+			$this->synchronized(function() : void{
+				while($this->unstack() !== null);
+			});
 			$this->notify();
 			$this->shutdown();
 		}