Fix longest chain finalization target lookup (#13289)

* Finalization target should be chosed as some ancestor of SelectChain::best_chain

* More test assertions

* Improve docs

* Removed stale docs

* Rename 'target' to 'base' in lookup method

* Fix typo

* Apply suggestions from code review

Co-authored-by: André Silva <[email protected]>

* Rename 'target_hash' to 'base_hash' in 'SelectChain::finality_target()'

* Apply suggestions from code review

Co-authored-by: Anton <[email protected]>

* Docs improvement

* Doc fix

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <[email protected]>

* Apply more code suggestions

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Co-authored-by: André Silva <[email protected]>
Co-authored-by: Anton <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>