Optimize collator-protocol advertisement fetching rule

[rphmeier]

From https://github.com/paritytech/polkadot/issues/5054#issuecomment-1105711569 :

We could relax this somewhat to fetch & second collations that are built on top of a Seconded candidate which we've validated locally, but that'd be more complex and the 'backed' rule should work OK although it's not as efficient.

The initial collator-protocol implementation for asynchronous backing won't be fully optimized, so by doing this we can theoretically get more throughput.