make the approxQuantiles* method go thru the exactQuantiles* method, with a special interpolation_type=-1 to cause a single pass best guess. This eliminates the replicated code that was in approxQuantiles and makes approxQuantiles better for the corner cases where you can actually get an exact answer. (shouldn't affect anything since R doesn't use the approx, and browser default is exact). although some code tweaks in exactQuantiles to make it work.
various summary2/quantile/test.0xdata.com tweaks. focused the xorsum tests to generate numbers away from the known bugs, to see if there were issues in other number ranges
Merge branch 'master' of https://github.com/0xdata/h2o