HPCC-12391 Refactor activity timers to track start/end time too

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
It's measuring the time taken to deliver the first row. Your version would be equal to elapsed() - latency(), so either can be used to recover the other - but mine seemed more natural. 

If you can think of a better name than latency for this, let me know.