devnet: blocktree bug causes finality to fail (and possibly other things)

[noot]

Task summary

• on the devnet, I saw the following error:

2021-11-17T23:08:11Z WARN failed to play grandpa round: no common ancestor between two nodes: 0xf48c9edaf2da5ec19edb7ced855158d295ad00ed2108cec4c8e22a87e2b86d33 and 0xcf2b5f5b59def5e4df6ff2237cca1fda5e5d114c20e796a90a58ace1bd91c20d	grandpa.go:L346	pkg=grandpa

this error comes from the blocktree function HighestCommonAncestor which returns the common ancestor between two blocktree nodes with the highest number. however this error should NOT happen, as explained by the comment above the error:

	ancestor := an.highestCommonAncestor(bn)
	if ancestor == nil {
		// this case shouldn't happen - any two nodes in the blocktree must
		// have a common ancestor, the lowest of which is the root node
		return common.Hash{}, fmt.Errorf("%w: %s and %s", ErrNoCommonAncestor, a, b)
	}

when this function returns an error it causes grandpa to exit as it's an unexpected err, then finality stops.

fix the blocktree so this does not happen!!

[github-actions]

🎉 This issue has been resolved in version 0.6.0 🎉

The release is available on GitHub release

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