Use an ISO-8601 string for log timestamp
Use an ISO-8601 string for log timestamp

This simplifies the process of getting a timestamp for the log as well
as switching to using UTC times.

Fixes uProxy/uproxy#1650

jpevarnek/uproxy,jpevarnek/uproxy,uProxy/uproxy,jpevarnek/uproxy,uProxy/uproxy,jpevarnek/uproxy,uProxy/uproxy,uProxy/uproxy,jpevarnek/uproxy,uProxy/uproxy