- 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
- Only syncs block headers (excludes block bodies, transactions, receipts, and logs)
- Modifies validation window to include the most recent block
- Isolates validation window to the variable defined in the cmd directory (blocks
have a separate variable defined in the block_repository for determining when
to set a block as final)
