Follow up with attestation "seen" propagation

[paulhauner]

Description

In ethereum/consensus-specs#1749 I suggest a fix to the spec.

Over in #983 I've implemented my suggested fix without it being part of the spec:

lighthouse/beacon_node/beacon_chain/src/attestation_verification.rs

Lines 238 to 251 in 8ea5397

	// Ensure the aggregated attestation has not already been seen locally.
	//
	// TODO: this part of the code is not technically to spec, however I have raised a PR to
	// change it:
	//
	// https://github.com/ethereum/eth2.0-specs/pull/1749
	let attestation_root = attestation.tree_hash_root();
	if chain
	.observed_attestations
	.is_known(attestation, attestation_root)
	.map_err(|e| Error::BeaconChainError(e.into()))?
	{
	return Err(Error::AttestationAlreadyKnown(attestation_root));
	}

We should keep an eye on the spec PR and resolve this TODO eventually.

[paulhauner]

This issues is blocked on #983, it only becomes valid once it is merged.

[paulhauner]

Not relevant anymore