improve logging: generate unique nonce for each request for logging purposes

[jas4711]

To make it easier to parse the log files, the validation server
could generate a random nonce and use that in the logging for all
log lines for that request. We tried (but failed) to achieve
this by putting the otp and nonce in the log lines, but they
aren't guaranteed to be unique.

[jas4711]

This is not worth doing just to fix this -- however we could revamp the entire logging later on.