Avoid updating the block gap when it's unchanged (paritytech#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
paritytech#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`.

---------

Co-authored-by: Davide Galassi <davxy@datawok.net>

prdoc/pr_5540.prdoc

@@ -0,0 +1,12 @@
+title: Avoid unnecessary block gap updates
+
+doc:
+  - audience: Node Dev
+    description: |
+      Previously, the block gap storage in database and state in `BlockchainDb` could be updated even if no changes occurred.
+      This commit refines the logic to ensure updates only occur when the block gap value actually changes, reducing unnecessary
+      writes and enhancing overall efficiency.
+
+crates:
+  - name: sc-client-db
+    bump: none