P2P signature sync skips requests with no local signatures

[Trynax]

@oxsteins

I also noticed a small p2p sync recovery gap around signature requests that have no local signatures yet.
From what I can tell, BuildWantSignaturesRequest() currently scans requests via GetSignatureRequestsWithoutAggregationProof(), which is based on the aggregation-proof-pending index.

That works once the node has at least one local signature, because saving a signature creates the pending proof marker.
But if the node has the signature request and missed all signatures for it, there is no signature map yet and no proof-pending marker yet, so the request does not get included in the want-signatures request at all.

I was able to repro this locally with both badger and bbolt:

• save a validator set with 3 active validators
• save a signature request for that epoch
• save no signatures for the request
• call BuildWantSignaturesRequest()
• it comes back empty instead of asking for validator indices 0, 1, 2

I think the fix would be to make signature sync scan signature requests by epoch directly, then:

• if a signature map exists, ask for the missing indices like it does today
• if no signature map exists, load the validator set and ask for all active validator indices
• still skip aggregation-key requests if the aggregation proof already exists

[Trynax]

Gm @oxsteins bumping this...