Only hash block headers in the lowest-level block index code.
This design has a few benefits:
- failures are obvious, because the hash is not available,
- get_tip() returns a smaller object,
- we avoid re-hashing block headers multiple times.
These efficiency changes may be needed to support chain reorganisations,
multiple tips, and heavy query loads.
Move block header hashing from zebra-consensus to zebra-state.
Handle zebra-state AddBlock errors in zebra-consensus BlockVerifier.
Add unit tests for BlockVerifier state error handling.
Part of #428.