asix-sigma: rephrase sample memory iteration position and trigger check

Rephrase how the sample memory iteration position gets tracked, increment
after every event slot already. Update the "last seen sample" status more
often (an event slot can hold several sample items). Arrange for a period
of time where software will check sample data for trigger matches. This
improves the precision of the hardware provided trigger match location.

Do send hardware provided trigger locations to the session feed even if
the software check found no match on the data content. This covers user
initiated button presses (which can unblock the acquisition when the
application provided trigger condition never matches).

Note that this implementation does manage the window of supervision, but
does not yet check the sample values against the trigger condition. This
gets added later.