#27, #32 - Produce a sliding average for the current heart rate of the
workout session.
#27, #32 - Added some instrumentation to understand the nature of a
failed invocation of timer_cancel() against a recurring timer in
Verifier.  It seems that when timer_cancel() returns a failing return
code when invoked against a recurring timer the recurring timer
continues firing.  This is odd, as the expectation is that a failing
return code indicates that at most a single event is already in flight
when the timer is cancelled, but in any case, the recurring timer is
cancelled.