Blocks backfilling option - 0 Empty implementations

[abi87]

How this works

Backfilling is split in the following PRs

• [X] Introduce new interface as well as empty implementation to unlock compilation. Coreth side is https://github.com/ava-labs/coreth/pull/395
• [ ] Implement block retrieval in engine, via GetAncestors. See https://github.com/ava-labs/avalanchego/pull/2144
• [ ] Implement backfil handling in proposervm. Mind need to rebuild backfilled blocks list upon restart (see https://github.com/ava-labs/avalanchego/pull/2241)
• [ ] Implement backfil in coreth. See https://github.com/ava-labs/avalanchego/pull/2255
• [ ] Experiment with bandwith aware peer tracker to speed up blocks download. See https://github.com/ava-labs/avalanchego/pull/2273

How this was tested

CI + successful mainnet sync with https://github.com/ava-labs/avalanchego/pull/2273 code

[github-actions[bot]]

This PR has become stale because it has been open for 30 days with no activity. Adding the lifecycle/frozen label will cause this PR to ignore lifecycle events.