TopK optimization to store not more than k values per window

[arunkm]

[peterfreiling]

Please make sure to honor all style cop (Roslyn analyzers) and VS style guidelines (should be red lines in your environment). If you are using VS and not seeing them let's figure out why. If not please find a way to enforce them locally.

[peterfreiling]

I realize the old code could use better documentation but the new code you are adding is not trivial and needs to be properly commented/documented. Please take time to comment thoroughly, and improve existing code when appropriate.