AbstractServer: Store app instance as class property

This will make it easier to reuse the instance in an asynchronous
setting (e.g. ReactPHP), where one application instance is preloaded
and reused for each incoming request.

src/Foundation/AbstractServer.php

@@ -17,6 +17,11 @@ use Monolog\Logger;
 
 abstract class AbstractServer
 {
+    /**
+     * @var Application
+     */
+    protected $app;
+
     /**
      * @var string
      */
@@ -41,6 +46,8 @@ abstract class AbstractServer
         if (file_exists($file = $this->path.'/config.php')) {
             $this->config = include $file;
         }
+
+        $this->app = $this->getApp();
     }
 
     /**

src/Http/AbstractServer.php

@@ -22,12 +22,10 @@ abstract class AbstractServer extends BaseAbstractServer
 {
     public function listen()
     {
-        $app = $this->getApp();
-
-        $this->collectGarbage($app);
+        $this->collectGarbage($this->app);
 
         $server = Server::createServer(
-            $this->getMiddleware($app),
+            $this->getMiddleware($this->app),
             $_SERVER,
             $_GET,
             $_POST,