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.
What exactly is the point of saving and renaming as opposed to saving directly?