Merge branch '6.4' into 7.4

* 6.4:
  [FrameworkBundle] Cache the serializer metadata in debug mode when it comes from mapping files

# Conflicts:
#	src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php
#	src/Symfony/Bundle/FrameworkBundle/Tests/DependencyInjection/FrameworkExtensionTestCase.php