Server: Prevent construction of more than one server instance at a time

I have no idea when one might choose to do this, but it could cause extremely undesirable effects, so it should be prevented.

src/pocketmine/Server.php

@@ -1434,6 +1434,9 @@ class Server{
 	 * @param string          $pluginPath
 	 */
 	public function __construct(\ClassLoader $autoloader, \ThreadedLogger $logger, string $dataPath, string $pluginPath){
+		if(self::$instance !== null){
+			throw new \InvalidStateException("Only one server instance can exist at once");
+		}
 		self::$instance = $this;
 		self::$sleeper = new \Threaded;
 		$this->autoloader = $autoloader;