Elastic scaling: make sure to preserve candidate ordering in GetBackedCandidates

[alindima]

Here:

polkadot-sdk/polkadot/node/core/backing/src/lib.rs

Line 2232 in e2ead88

fn handle_get_backed_candidates_message(

We need to trim all candidates of the paraid that follow a candidate for which we couldn't retrieve its data. Otherwise, we may end up supplying invalid chains to the runtime. The runtime should trim this for us, but it'd be better to not supply invalid chains to the runtime in the first place

We can also double check this in the provisioner after we get a response