## Issue Addressed
- Resolves#3266
## Proposed Changes
Return 200 OK rather than an error when a block, attestation or sync message is already known.
Presently, we will log return an error which causes a BN to go "offline" from the VCs perspective which causes the fallback mechanism to do work to try and avoid and upcheck offline nodes. This can be observed as instability in the `vc_beacon_nodes_available_count` metric.
The current behaviour also causes scary logs for the user. There's nothing to *actually* be concerned about when we see duplicate messages, this can happen on fallback systems (see code comments).
## Additional Info
NA
## Issue Addressed
https://github.com/sigp/lighthouse/issues/3091
Extends https://github.com/sigp/lighthouse/pull/3062, adding pre-bellatrix block support on blinded endpoints and allowing the normal proposal flow (local payload construction) on blinded endpoints. This resulted in better fallback logic because the VC will not have to switch endpoints on failure in the BN <> Builder API, the BN can just fallback immediately and without repeating block processing that it shouldn't need to. We can also keep VC fallback from the VC<>BN API's blinded endpoint to full endpoint.
## Proposed Changes
- Pre-bellatrix blocks on blinded endpoints
- Add a new `PayloadCache` to the execution layer
- Better fallback-from-builder logic
## Todos
- [x] Remove VC transition logic
- [x] Add logic to only enable builder flow after Merge transition finalization
- [x] Tests
- [x] Fix metrics
- [x] Rustdocs
Co-authored-by: Mac L <mjladson@pm.me>
Co-authored-by: realbigsean <sean@sigmaprime.io>
