Fixed AsyncTask memory leak (#922)

* Added PocketMine-TesterPlugin submodule with regression test

src/pocketmine/scheduler/AsyncPool.php

@@ -158,6 +158,10 @@ class AsyncPool{
 			}
 		}
 
+		foreach($this->workers as $worker){
+			$worker->collect();
+		}
+
 		Timings::$schedulerAsyncTimer->stopTiming();
 	}
 }

src/pocketmine/scheduler/AsyncTask.php

@@ -261,7 +261,8 @@ abstract class AsyncTask extends Collectable{
 				$this->{$p} = null;
 			}
 		}
-	}
 
+		$this->setGarbage();
+	}
 }