Fit: use lap start time instead of end

some files I checked had bogus end times in Laps - while the start times
were fine. So I changed the code to caluclate the end time (and not the
start time)
Fit: debug-log more unhandled message details
Info: provide total recorded time

... and use this for correct calculation of average chunk duration.
Fit: refactored chunk start calculation

lost track how chnuk start is actually calculated - refactored it and took
more corner cases into consideration.