lookahead collator: only build on top of scheduled relay parents (#1429)
Partially addresses #1400
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Co-authored-by: 
Andrei Sandu <54316454+sandreim@users.noreply.github.com>
parent
730edeb6
Partially addresses #1400
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Co-authored-by: Andrei Sandu <54316454+sandreim@users.noreply.github.com>