Support Interval and Utc time in our logging channels

This change adds support for both Interval time (system up time)
and Utc time (in millis).  The former column will always be present
but the latter is only available after we have synced with a reliable
time source (GPS, Cell Network).

Only need to change how we handle grabbing channel samples so that
we don't get any clipping that would be introduced by casting from
int to float like we currently do.