State roots cannot be indexes by the chunk hash

[SkidanovAlex]

The state root after chunk application depends on the incoming receipts, which are not specific to any particular chunks. A chunk included on two different forks (at the same or at different heights) can have different incoming receipts when it is processed.

There are several possible solutions, two that come to mind:

1. Index by (chunk_hash, block_hash)
2. Index by (block_hash, shard_id)

Without this change a chunk validation might fail if the same chunk was processed on a different fork and saved an inconsistent state root.

cross_shard_tx test with the block production time configured sufficiently low reproduces this bug with ~25% chance.