Implements COUNT/SUM/AVG(DISTINCT) and SELECT DISTINCT for materialized
views. To do this we have to keep a list of the actual distinct values
(similarly to how we do for min/max). We then update the operator (and
issue deltas) only when there is a state transition (for example, if we
already count the value x = 1, and we see an insert for x = 1, we do
nothing).
SELECT DISTINCT (with no aggregator) is similar. We already have to keep
a list of the values anyway to power the aggregates. So we just issue
new deltas based on the transition, without updating the aggregator.
Closes#3808
f85ba9198f