Add self-consistency check to attestation gossip validation

specs/phase0/p2p-interface.md

@@ -403,6 +403,8 @@ The following validations MUST pass before forwarding the `attestation` on the s
 - _[REJECT]_ The current `finalized_checkpoint` is an ancestor of the `block` defined by `attestation.data.beacon_block_root` -- i.e.
   `get_ancestor(store, attestation.data.beacon_block_root, compute_start_slot_at_epoch(store.finalized_checkpoint.epoch))
   == store.finalized_checkpoint.root`
+- _[REJECT]_ The attestation's epoch matches its target -- i.e. `attestation.data.target.epoch ==
+  compute_epoch_at_slot(attestation.data.slot)`