consensus: future-iteration Quorum is ignored

[fed-franz]

Summary

In the current implementation, nodes discard Quorum messages from future iterations in the same round. However, this could make the node fall out of sync, potentially missing generation-vote assignments on the next round.

Similar to Quorum from future rounds, we might have nodes request the missing candidate and accept the block once it's full (candidate+attestation).

Detailed Description

Current-round Quorum messages are forwarded by the Acceptor to the Consensus (event_loop) where they get discarded due to not meeting current criteria (if qround != round || qprev != prev || qiter > iter).

Relevant Context

Future-round Quorum messages currently triggers a Candidate request and are not forwarded to Consensus.
This constitutes a discrepancy with future-iteration message which should be re-evaluated to assert if there is indeed any rationale for it.
If not the case, than future-iteration Quorum messages should be treated as future-round ones.

Possible Solution

Merge future-iteration case with future-round case: if a future-iteration Quorum message is received, it should trigger a Candidate block request (if missing) and then try to build and accept the full block.

OPTIONAL: given such a message would be from the current round, it is possible to verify its signature against the corresponding committee before requesting the candidate.