initial integration of attestation pool with fork choice infrastructure

[tersec]

Of all things, this doesn't actually aggregate. I don't see how that's supposed to happen upstream of get_latest_attestation, which is the only part this addresses.

This does integrate changes with @zah's minimal attestation pool in fork_choice, unlike the previous branch. I'm not entirely convinced this is better, as an architectural design -- it intermixes fork choice and other attestation pool mechanics, unlike https://github.com/status-im/nim-beacon-chain/tree/WorkPoolUpdates for example, rather than layering them -- but, sure, I'll give this a try.

attestation_participants in AttestationCandidate would be for use in aggregating, which, see above, I'm not 100% convinced can live in this layer anyway, consistently with spec definitions.

[zah]

This should have been done on top of the network-sim branch (now, merged in master)