Speed up CDB_HexagonGrid
Speed up CDB_HexagonGrid

With this commit the test grid (25690 cells covering webmercator
extent) builds in 0.2 seconds rather than 9.4. It could be speeded
up more but I don't think it's needed yet...

datapolitan/cartodb20,datapolitan/cartodb20,datapolitan/cartodb20,jmwenda/nationmaps,jmwenda/nationmaps,datapolitan/cartodb20,jmwenda/nationmaps,jmwenda/nationmaps