Register MainLogger on AsyncWorkers to make MainLogger::getLogger() usable in AsyncTasks

Threaded static properties aren't thread-local anymore in pthreads 3.1.7dev

src/pocketmine/scheduler/AsyncTask.php

@@ -25,6 +25,7 @@ namespace pocketmine\scheduler;
 
 use pocketmine\Collectable;
 use pocketmine\Server;
+use pocketmine\utils\MainLogger;
 
 /**
  * Class used to run async tasks in other threads.