Use FilesystemCache from Doctrine\Common\Cache

Directly using a FilesystemCache from Doctrine\Common\Cache.

I added the filemtime to the cacheKey. This will cause mor stats on the files, but hopefully phps statcache will solve this.

I think these are the minimal changes to use Doctrine\Common\Cache. I think it would be better to pass a Cache implementation to the constructor but this would also cause a BC break, which I tried not to do.