Avoid updating the block gap when it's unchanged (#5540)

There are basically three commits in this PR. Since all these commits
essentially have no logical changes, I packed them into one PR. Review
by per-commit is recommended.
- The first commit avoids unnecessarily updating the block gap storage
when the value remains unchanged, as discovered when I worked on
https://github.com/paritytech/polkadot-sdk/issues/5406.
- The second commit is purely about format string style changes but
deletes ~10 lines of code, which slightly helps me look into this file
:P
- The third commit is added to avoid the unnecessary block gap update in
`BlockchainDb`.

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Co-authored-by: Davide Galassi <[email protected]>