add timestamp to diagnostic log entries

[vladak]

When using the diagnostic log to track down problems with an IdP, I found that it would benefit from having time stamps written to the log, somewhere around here: https://github.com/latchset/mod_auth_mellon/blob/b1e30ff4ae684341a96f41619d58358b2044784e/auth_mellon_diagnostics.c#L705-L706 and here https://github.com/latchset/mod_auth_mellon/blob/b1e30ff4ae684341a96f41619d58358b2044784e/auth_mellon_diagnostics.c#L767

[3eka]

+1

[thijskh]

A pull request would be very welcome!