standardize code coverage

[stevenolen]

At the moment our code coverage and testing story is weird/unkept. We should endeavor to adjust the codecov config to accomplish:

1. diminish tracking of ptd/ptd python-based coverage
2. ensure go paths are tracked
3. set a good minimum coverage percentage and actually reject PR merges if they cause it to drop below
4. set a good minimum change for a given PR so one-liners or dumb changes don't fail cov test