Integrate resultsdb with the backend.

This changeset should accomplish the needs of issue #101.

- [x] when a stack is created, cascade
- [x] when a package is created, cascade
- [x] when an update is created, cascade
- [x] write tests for all that cascading stuff
- [x] do hub checks at push time to make sure required bits are happy (gating)
- [x] write tests for hub check
- [x] do app checks at request-for-stable time to make sure required bits are happy too
- [x] write tests for request-for-stable check