refactor to speed up case of Exec quantile passing long list of thresholds (and future Summary2). Since the first pass qbin is the same for all thresholds, save it, and start with it for all thresholds. Since many times the exact answers don't require >1 pass, this speeds things up a lot. Cases that need more passes for the exact answer will still iterate..they just start with pass 2 if needed. Will need to move the exec code into a another file, along with the QuantilesPage stuff (which supports exact or approx)..but delay on that (that refactor may also support Summary2 multi-threshold, although Summary2 does a double binning (browser view + 1000 binning, in one pass, so maybe want to keep that to avoid two passes there. Tests pass.

customer will potentially use long list of thresholds to exec quantile (100?), to get levels for balanced bins. If that can use the approximation method, we could guarantee it to be just one pass for speed, but I think the multi-capable wil
l be fine (and exact)