Change `store::on_gossip_attestation` take a reference to signed attestations

[MegaRedHand]

The function currently takes ownership of the attestation:

ethlambda/crates/blockchain/src/store.rs

Lines 357 to 360 in bae9af5

	pub fn on_gossip_attestation(
	store: &mut Store,
	signed_attestation: SignedAttestation,
	) -> Result<(), StoreError> {

This could be changed to receive a reference, to avoid copying the signature. Internally we move the attestation data, but that is easily clonable, while the signature is not.