Consider distributing incoming receipts outside of ChunkStateWitness

[jancionear]

Incoming receipts to a single chunk could potentially get large - if every shard sends out a few megabytes of receipts to a single shard, it could quickly add up to tens of megabytes in ChunkStateWitness::source_receipt_proofs.

But we already have a mechanism for distributing incoming receipts - maybe we could use it to send incoming receipts straight to the validators, without including them in ChunkStateWitness. The ChunkStateWitness would contain only the receipt proofs, without the actual receipts.