Implement sparse store

[Sytten]

In branch https://github.com/GraphMetrics/ddsketch-go/tree/feat/sparse-store
What is left:

• We might want a better growth strategy for the map than the default size doubling. Maybe this won't be possible as the internal map structure un golang is quite sophisticated on how it changes its internal size even though the result seems pretty dumb (doubling of size). This article explains well how it works: https://hackernoon.com/some-insights-on-maps-in-golang-rm5v3ywh
• Uniform collapsing strategy based on UDDsketch