#6435 Race Condition in Filesystem-Storage-Adapter

pull request regarding #6435
Url to repository: https://github.com/david-sann/zf2.git
branch zf6435

added file_exits to an if condition in Zend\Cache\Storage\Adapter\Filesystem:prepareDirectoryStructure()
to avoid a race condition if there are lots of parallel requests (for more information please see the issue)