output/csv: complete and improve timestamp construction support

output/csv: complete and improve timestamp construction support

Unbreak the timestamp calculation when session data is received in
multiple packets. Avoid a division by zero when the samplerate is not
known yet the time column is requested. Only calculate timestamps when
the time column is requested. Use floating point during the scaling,
only convert to integer immediately before printing. Change line breaks
to not split a complex sub-expression across several lines.