[Filesystem Component] mkdir race condition fix #11626
Merge branch '2.3' into 2.4

* 2.3:
  [HttpKernel] add use statement for phpdoc
  Disabled the PHPUnit self-update on Travis
  [ClassLoader] simplified phpdoc
  [ClassLoader] Add a __call() method to XcacheClassLoader
  fix some minor typos in tests
  [Yaml] fixed mapping keys containing a quoted #
  Added fixture to test parsing of hash keys ending with a space and #
  [Filesystem Component] mkdir race condition fix #11626
  [Validator] reverted permissions change on translation files
  Fixed Factory services not within the ServiceReferenceGraph.
  [CssSelector] Fix URL to SimonSapin/cssselect repo
  [Validator] Fixed wrong translation keys/messages for Collection constraint. The error messages for a missing field and an unexpected field did not match the Contraint class
  [YAML] resolve variables in inlined YAML
  Disallow abstract definitions from doctrine event listener registration

Conflicts:
	src/Symfony/Bridge/Doctrine/DependencyInjection/CompilerPass/RegisterEventListenersAndSubscribersPass.php
	src/Symfony/Bridge/Doctrine/Tests/DependencyInjection/CompilerPass/RegisterEventListenersAndSubscribersPassTest.php
	src/Symfony/Bundle/FrameworkBundle/EventListener/SessionListener.php
	src/Symfony/Component/Filesystem/Filesystem.php