"fast coverage" logic

- rather than fetching the entire feature polygon, reuse the broken up parts of the polygon in the s2 cells that we know we overlap with
  - we know every s2 cell we hit, and every polygon part in it
  - compute the sum of our overlaps with those pieces and divide that by the total size of the feature (new in this index)
- also, remove the fact that we were running our polygon computations twice
- also, remove the fact that we were running our coverage computations twice

Auditors: nsanch, rahul, daniel