TYPO3 Core

The null backend is very useful to effectivly disable caching
during development. With this change it also implements the
PhpCapableBackend interface and can be used to replace the
FileBackend this way.

This is the v4 backport of FLOW3 changeset
https://review.typo3.org/#change,3910

Additional to the FLOW3 version the nullbackend is now
required in config_default.php to enable the cache_phpcode
to use this backend in autoloader in development context.