traceapp: use math/big to calculate dashboard statistics
traceapp: use math/big to calculate dashboard statistics

This lets us avoid integer overflow due to the very large numbers ending up
in a time.Duration (int64) variable. Alternatively we could divide into a
lower resolution measurement (e.g. milliseconds or seconds), but this seems
generally more versatile.

bg451/appdash,joeshaw/appdash,bg451/appdash,bg451/appdash,joeshaw/appdash,joeshaw/appdash