## 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
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