Older versions of lteenb do not implement .utc field in remote API
reponses, only .time which is relative to the start of the process.
For such versions, inject an approximation of .utc into each logged
message.

Without this, KPI computation fails because .utc is missing.

Take `utc0 = time.now()` as an approximation of the .utc time when
receiving the first message after (re)connecting, and then for each
subsequent message compute `utc = utc0 - time0 + time`.

This preserves the relative time interval between each message.