minor #35109 [FrameworkBundle] cache ClassMetadataFactory in debug (bastnic)

This PR was merged into the 5.1-dev branch.

Discussion
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[FrameworkBundle] cache ClassMetadataFactory in debug

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no (performance)
| New feature?  | performance...
| Deprecations? | no
| Tickets       | #35096
| License       | MIT
| Doc PR        |

Cf #35096, validator and serialization yaml/xml files are parsed at each requests. But they are already tracked by a `DirectoryResource` in
bd9dc7c573/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php (L1246-L1261)

So it only needs a real cache purge on validator / serializer sytem cache, and I use the [CachePoolClearerCacheWarmer](https://github.com/api-platform/core/blob/master/src/Bridge/Symfony/Bundle/CacheWarmer/CachePoolClearerCacheWarmer.php) from APIP that already does similar things.

Handling that sort of cache in Symfony fixes a lot of others performance issue.

![image](https://user-images.githubusercontent.com/84887/71452294-e6869480-2783-11ea-9aa8-431db3edfd8d.png)

Commits
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53324986cb [FrameworkBundle] cache ClassMetadataFactory in debug

src/Symfony/Bundle/FrameworkBundle/CacheWarmer/CachePoolClearerCacheWarmer.php

@@ -0,0 +1,57 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Bundle\FrameworkBundle\CacheWarmer;
+
+use Symfony\Component\HttpKernel\CacheClearer\Psr6CacheClearer;
+use Symfony\Component\HttpKernel\CacheWarmer\CacheWarmerInterface;
+
+/**
+ * Clears the cache pools when warming up the cache.
+ *
+ * Do not use in production!
+ *
+ * @author Kévin Dunglas <dunglas@gmail.com>
+ *
+ * @internal
+ */
+final class CachePoolClearerCacheWarmer implements CacheWarmerInterface
+{
+    private $poolClearer;
+    private $pools;
+
+    public function __construct(Psr6CacheClearer $poolClearer, array $pools = [])
+    {
+        $this->poolClearer = $poolClearer;
+        $this->pools = $pools;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function warmUp($cacheDirectory): void
+    {
+        foreach ($this->pools as $pool) {
+            if ($this->poolClearer->hasPool($pool)) {
+                $this->poolClearer->clearPool($pool);
+            }
+        }
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function isOptional(): bool
+    {
+        // optional cache warmers are not run when handling the request
+        return false;
+    }
+}

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

@@ -1457,10 +1457,6 @@ class FrameworkExtension extends Extension
         $chainLoader->replaceArgument(0, $serializerLoaders);
         $container->getDefinition('serializer.mapping.cache_warmer')->replaceArgument(0, $serializerLoaders);
 
-        if ($container->getParameter('kernel.debug')) {
-            $container->removeDefinition('serializer.mapping.cache_class_metadata_factory');
-        }
-
         if (isset($config['name_converter']) && $config['name_converter']) {
             $container->getDefinition('serializer.name_converter.metadata_aware')->setArgument(1, new Reference($config['name_converter']));
         }

src/Symfony/Bundle/FrameworkBundle/Resources/config/cache_debug.xml

@@ -11,5 +11,15 @@
         <service id="data_collector.cache" class="Symfony\Component\Cache\DataCollector\CacheDataCollector" public="true">
             <tag name="data_collector" template="@WebProfiler/Collector/cache.html.twig" id="cache" priority="275" />
         </service>
+
+        <!-- CacheWarmer used in dev to clear cache pool -->
+        <service id="cache_pool_clearer.cache_warmer" class="Symfony\Bundle\FrameworkBundle\CacheWarmer\CachePoolClearerCacheWarmer" public="false">
+            <argument type="service" id="cache.system_clearer" />
+            <argument type="collection">
+                <argument>cache.validator</argument>
+                <argument>cache.serializer</argument>
+            </argument>
+            <tag name="kernel.cache_warmer" priority="64" />
+        </service>
     </services>
 </container>

src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/FrameworkExtensionTest.php

@@ -1102,10 +1102,10 @@ abstract class FrameworkExtensionTest extends TestCase
         $this->assertEquals(new Reference('serializer.mapping.cache.symfony'), $cache);
     }
 
-    public function testSerializerCacheDisabled()
+    public function testSerializerCacheActivatedDebug()
     {
         $container = $this->createContainerFromFile('serializer_enabled', ['kernel.debug' => true, 'kernel.container_class' => __CLASS__]);
-        $this->assertFalse($container->hasDefinition('serializer.mapping.cache_class_metadata_factory'));
+        $this->assertTrue($container->hasDefinition('serializer.mapping.cache_class_metadata_factory'));
     }
 
     public function testSerializerMapping()