Add gossip conditions from spec v0.12.3 (#1667)

## Issue Addressed

NA

## Proposed Changes

There are four new conditions introduced in v0.12.3:

 1. _[REJECT]_ The attestation's epoch matches its target -- i.e. `attestation.data.target.epoch ==
  compute_epoch_at_slot(attestation.data.slot)`
1. _[REJECT]_ The attestation's target block is an ancestor of the block named in the LMD vote -- i.e.
  `get_ancestor(store, attestation.data.beacon_block_root, compute_start_slot_at_epoch(attestation.data.target.epoch)) == attestation.data.target.root`
1. _[REJECT]_ The committee index is within the expected range -- i.e. `data.index < get_committee_count_per_slot(state, data.target.epoch)`.
1. _[REJECT]_ The number of aggregation bits matches the committee size -- i.e.
  `len(attestation.aggregation_bits) == len(get_beacon_committee(state, data.slot, data.index))`.

This PR implements new logic to suit (1) and (2). Tests are added for (3) and (4), although they were already implicitly enforced.

## Additional Info

- There's a bit of edge-case with target root verification that I raised here: https://github.com/ethereum/eth2.0-specs/pull/2001#issuecomment-699246659
- I've had to add an `--ignore` to `cargo audit` to get CI to pass. See https://github.com/sigp/lighthouse/issues/1669

beacon_node/network/src/beacon_processor/worker.rs

@@ -847,6 +847,32 @@ impl<T: BeaconChainTypes> Worker<T> {
                 );
                 self.penalize_peer(peer_id.clone(), PeerAction::LowToleranceError);
             }
+            AttnError::InvalidTargetEpoch { .. } => {
+                /*
+                 * The attestation is malformed.
+                 *
+                 * The peer has published an invalid consensus message.
+                 */
+                self.propagate_validation_result(
+                    message_id,
+                    peer_id.clone(),
+                    MessageAcceptance::Reject,
+                );
+                self.penalize_peer(peer_id.clone(), PeerAction::LowToleranceError);
+            }
+            AttnError::InvalidTargetRoot { .. } => {
+                /*
+                 * The attestation is malformed.
+                 *
+                 * The peer has published an invalid consensus message.
+                 */
+                self.propagate_validation_result(
+                    message_id,
+                    peer_id.clone(),
+                    MessageAcceptance::Reject,
+                );
+                self.penalize_peer(peer_id.clone(), PeerAction::LowToleranceError);
+            }
             AttnError::TooManySkippedSlots {
                 head_block_slot,
                 attestation_slot,