Because time.Time doesn't work correctly with our YAML package, it is necessary
to introduce a type, util.Time, which serializes correctly to JSON and YAML.
Eventually we would like timestamping to cut across storage implementations;
for now, we set it in each storage.
Specifying an API type as IntOrString will allow JSON and YAML to accept
either ints or strings with the same name. For example, port names or
numbers.
