Fix missing strFailReason in CreateTransaction (Russell Yanofsky)
Fix misplaced AssertLockHeld (Russell Yanofsky)
doc: Clarify WalletStorage / Wallet relation (Russell Yanofsky)
Add back mistakenly removed AssertLockHeld (Russell Yanofsky)
Refactor: Add GetLegacyScriptPubKeyMan helper (Russell Yanofsky)
Pull request description:
This PR implements suggested code cleanups from https://github.com/bitcoin/bitcoin/pull/17260 review comments
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https://github.com/bitcoin/bitcoin/pull/17300/files
Depends on D7119
Backport of Core PR17300