MINOR: activity: report profiling duration and age in "show profiling"

MINOR: activity: report profiling duration and age in "show profiling"

Seeing counters in "show profiling" is not always very helpful without
an indication of how long the analysis lasted nor if it's still active
or not. Let's add a pair of start/stop timers for tasks and memory so
that we can now indicate how long the measurements lasted and when they
ended (or 0 if still running).

Note that for tasks profiling set to "auto", the measurement is considered
enabled since it can automatically switch on and off on a per-thread
basis.