Check for parent of first ready block being on chain (#1812)

When retrieving the ready blocks, verify that the parent of the first
ready block is on chain. If the parent is not on chain, we are
downloading from a fork. In this case, keep downloading until we have a
parent on chain (common ancestor).

Resolves https://github.com/paritytech/polkadot-sdk/issues/493.

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Co-authored-by: Aaro Altonen <48052676+altonen@users.noreply.github.com>