Request availability chunks from actual backers

[eskimor]

With paritytech/polkadot#2423 in place we are requesting availability chunks from validators in the backing group of that candidate. Problem is not all of them will be actual backers, which results in needless requests. Either in parallel - wasting bandwidth or in sequence on failure - wasting time.

Solutions:

1. Either record actual backers on chain in OccupiedCore in a backer bitfield for example. We would need to do some unification of backer bitfield (bitwise OR) because different subsets of backers might appear in different chains and we'd want to request from anyone who signed a statement.
2. Backers send out notifications via gossip or directly once they backed a candidate.
3. Collect that information from statement distribution already - backers already gossip, that they backed a candidate .. that should work as well, but I will need to read up on statement distribution first.

[eskimor]

Thanks @ordian - was just searching for the right tags.