Fix race condition in fileserver cache

I was seeing this on the 2014.1 branch, which didn't have the try/except block that hides the failures merged in, This uses os.mkdir which is hopefully atomic and cross-platform. I'd like to thank http://pythonhosted.org/lockfile/ for that idea. It might be better to actually just use that lockfile implementation, but I didn't want to unilaterally introduce a new dependancy.