SketchMap#create improvements

[andrewnguyen]

• Create heavyHitters, totalValue and table all at once instead of one by one
• Instead of creating a new intermediate SketchMap for every k,v pair, only create one SketchMap for the whole sequence of data

[andrewnguyen]

• Creating a SketchMap from a large sequence of key value pairs can be slow #225

[johnynek]

any benchmarks here? Thanks for doing this.

[andrewnguyen]

I don't have any comprehensive benchmarks saved, but the one I did was around ~4-10x speedup. (basically an order of magnitude change)