atomic tx TODOs

[loredanacirstea]

• atomic block proposer should have access to all the chains involved; right now we just choose the leader chain, but we need to put this limitation on the proposer too
• the propose block phase now takes longer because the proposer executes the atomic tx; I mitigated this by extending the propose timeout, but we only should do this if there is an atomic tx in the proposed block; so the propose phase should be normal, but the proposer will then execute and send another proposal to the other nodes, with additional MetaInfo about the cross-chain requests & results. the other nodes will have an additional protocol phase, to wait for this additional data before trying to finalize the block.