Add an config option to use a "scalable" profile helper for edge counters,
where we try and avoid contention by probablistic updates once the counter
value exceeds some threshold (currently 8192). Using the current xorshift
RNG this gives the counter a two-sigma accuracy of around +/- 2%.

The idea is loosely based on "Scalable Statistics Counters" by Dice, Lev,
and Moir (SPAA’13).

Also allow the scalable and interlocked profile modes to operate at the same
time, recording two sets of counts per probe, so we can verify that this new mode
is sufficiently accurate.