Use PSR-15 middleware standard

This finally adopts the new standardized interfaces instead of the
work-in-progress ones with the `Interop\` prefix.

Since we have now updated to PHP 7.1, we can also use Stratigility
3.0 as the middleware dispatcher.

src/Http/Middleware/RememberFromCookie.php

@@ -13,11 +13,12 @@ namespace Flarum\Http\Middleware;
 
 use Flarum\Http\AccessToken;
 use Flarum\Http\CookieFactory;
-use Interop\Http\ServerMiddleware\DelegateInterface;
-use Interop\Http\ServerMiddleware\MiddlewareInterface;
+use Psr\Http\Message\ResponseInterface as Response;
 use Psr\Http\Message\ServerRequestInterface as Request;
+use Psr\Http\Server\MiddlewareInterface as Middleware;
+use Psr\Http\Server\RequestHandlerInterface as Handler;
 
-class RememberFromCookie implements MiddlewareInterface
+class RememberFromCookie implements Middleware
 {
     /**
      * @var CookieFactory
@@ -32,7 +33,7 @@ class RememberFromCookie implements MiddlewareInterface
         $this->cookie = $cookie;
     }
 
-    public function process(Request $request, DelegateInterface $delegate)
+    public function process(Request $request, Handler $handler): Response
     {
         $id = array_get($request->getCookieParams(), $this->cookie->getName('remember'));
 
@@ -48,6 +49,6 @@ class RememberFromCookie implements MiddlewareInterface
             }
         }
 
-        return $delegate->process($request);
+        return $handler->handle($request);
     }
 }