This diff addresses the following points of https://github.com/ooni/probe/issues/2135:
- [x] the `childResolver` type is useless and we can use `model.Resolver` directly;
- [x] we should use `model/mocks` instead of custom fakes;
- [x] we should not use `log.Log` rather we should use `model.DiscardLogger`;
- [x] make `timeLimitedLookup` easier to test with a `-short` tests;
- [x] ensure `timeLimitedLookup` returns as soon as its context expires regardless of the child resolver;
Subsequent diffs will address more points mentioned in there.
* feat(sessionresolver): try many and use what works
* fix(sessionresolver): make sure we can use quic
* fix: the config struct is unnecessary
* fix: make kvstore optional
* feat: write simple integration test
* feat: start adding tests
* feat: continue writing tests
* fix(sessionresolver): add more unit tests
* fix(sessionresolver): finish adding tests
* refactor(sessionresolver): changes after code review
