DAL: attestations's level has same meaning as for consensus attestations

Before this MR, the level of an attestation referred to the level where it was supposed to be included in a block (the block at level published_level + attestation_lag). However, merging DAL attestations with consensus attestation would then be problematic, because the level of a consensus operation included at level n is n-1. With this MR, the level of a DAL attestation is as for consensus operations.

Note that to attest a slot at published at level published_level, one should therefore inject an DAL attestation with level published_level + attestation_lag - 1.