chain_start can never be null, so it's better to pass it as a reference
rather than a raw pointer
Also slightly reformat HeaderSyncState constructor to make clang-format
happy
Lastly, remove `const` from `chain_start` declaration in
headers_sync_chainwork_tests, to work aroud a false-positive
dangling-reference warning in gcc 13.0
Co-Authored-By: maflcko <6399679+maflcko@users.noreply.github.com>
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