xor the random number seed with the pid

[waltman]

This prevents 2 processes starting at the same microsecond (e.g. after a fork) from generating the same UUID. This fixes the bug reported at https://blog.afoolishmanifesto.com/posts/log-loss-detection/

[waltman]

I tested this using the shell command frew used in his blog post, but I don't know how do write it within Perl's testing framework.