handle the new sign_attestations more efficiently #819
Description
In #813, we updated to Lighthouse 8.1.0.
In there is batched signing of attestations via sign_attestations. We currently just delegate to sign_attestation in its previous form. However, as the actual steps (deciding on a vote and collecting the signatures) are preformed on a per committee basis, we can optimize it by performing those steps once per committee. This avoids redundant futures.