Always use read/write copy method instead of fileutils.copy

Fileutils.copy seems to be doing a lot of weird stuff which ends up making it a lot slower in my testing than the current "fallback" (nfs) method is.

Here's a small example program that I used in testing: https://gist.github.com/jbylund/6ca123b988996bcae3c1517d8b98c7ff

In my experimentation the fallback (or a lightly modified version thereof) takes ~1/2 as long.