Align aggregated att gossip validations

[terencechain]

This PR implements ethereum/consensus-specs#3552 by adding the following gossip verifications for aggregated attestation:

- [REJECT] The committee index is within the expected range -- i.e. `aggregate.data.index < get_committee_count_per_slot(state, aggregate.data.target.epoch)`.
- [REJECT] The number of aggregation bits matches the committee size -- i.e. len(aggregate.aggregation_bits) == len(get_beacon_committee(state, aggregate.data.slot, aggregate.data.index)).
- [REJECT] The aggregate attestation has participants -- that is, len(get_attesting_indices(state, aggregate.data, aggregate.aggregation_bits)) >= 1.

The check to ensure the 'committee index is within the expected range' has been moved to occur after the retrieval of the target state, to prevent a denial of service attack on the 'get target state' function.