* Return PopulateMissingHeaders early if the sync is at the head of the chain
* Squelch logging if no blocks to sync
* Fix broken test
* Refactor repository tests
- currently not validating price feeds if the underlying header already exists
and is valid, since price feeds should have been added when initial header
was added
- Migrate various mocks of core namespaces to shared version in `fakes` pkg
- Err on the side of making test doubles less sophisticated
- Don't pull over mocks of namespaces that are only used in example code
