Move garbage collection into middleware

This prevents garbage collection to randomly break the installer:
before installation, the models that are being accessed have no
database connection.

Now, the middleware is only mounted into the forum's middleware
stack. I want API requests to have stable performance, and the
forum middleware stack is only mounted when Flarum is installed.

src/Http/Middleware/CollectGarbage.php

@@ -0,0 +1,55 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Http\Middleware;
+
+use Flarum\Http\AccessToken;
+use Flarum\User\AuthToken;
+use Flarum\User\EmailToken;
+use Flarum\User\PasswordToken;
+use Psr\Http\Message\ResponseInterface as Response;
+use Psr\Http\Message\ServerRequestInterface as Request;
+use Zend\Stratigility\MiddlewareInterface;
+
+class CollectGarbage implements MiddlewareInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function __invoke(Request $request, Response $response, callable $out = null)
+    {
+        $this->collectGarbageSometimes();
+
+        return $out ? $out($request, $response) : $response;
+    }
+
+    private function collectGarbageSometimes()
+    {
+        // In order to save performance, we only execute this query
+        // from time to time (with 50% chance).
+        if (! $this->hit()) {
+            return;
+        }
+
+        AccessToken::whereRaw('last_activity <= ? - lifetime', [time()])->delete();
+
+        $earliestToKeep = date('Y-m-d H:i:s', time() - 24 * 60 * 60);
+
+        EmailToken::where('created_at', '<=', $earliestToKeep)->delete();
+        PasswordToken::where('created_at', '<=', $earliestToKeep)->delete();
+        AuthToken::where('created_at', '<=', $earliestToKeep)->delete();
+    }
+
+    private function hit()
+    {
+        return mt_rand(1, 100) <= 2;
+    }
+}