Utils: Make kill() use static MainLogger instead of global variable

src/pocketmine/utils/MainLogger.php

@@ -70,6 +70,14 @@ class MainLogger extends \AttachableThreadedLogger{
 		return static::$logger;
 	}
 
+	/**
+	 * Returns whether a MainLogger instance is statically registered on this thread.
+	 * @return bool
+	 */
+	public static function isRegisteredStatic() : bool{
+		return static::$logger !== null;
+	}
+
 	/**
 	 * Assigns the MainLogger instance to the {@link MainLogger#logger} static property.
 	 *