Fixed race conditions in FilesystemCache

In our environment (presumably because of the NFS) under high load after deployment, a lot of E_WARNINGs are emitted  because of race condition in mkdir() used in the FileSystemCache::save.

Fortunately I've seen a very good solution for this problem in the Twig library: https://github.com/fabpot/Twig/blob/master/lib/Twig/Environment.php :: writeCacheFile

I have copied and slightly adapted the cache write routine from Twig library. Hopefully it is allowed, with proper appreciation.
Why were these variables arbitrarily renamed? Makes the diff only harder to read...