DAL: how to handle multiple attestations from same attester in a block

Currently, all attestations from the same attestor and same level are applied, AFAIU. I think that should not be allowed (it allows spamming for instance?).

I guess we should allow:

• only one attestation (per pkh, level) in a block
• better attestations (that is, with more attested slots) to replace worse attestations in the mempool.