Add missing validator for database.useWAL

[DL6ER]

What does this implement/fix?

#1773 recently added config validation enforcing every config option to have a validator assigned to it. After this PR was created, another #1868 added the new option database.useWAL which is now lacking such a validator, causing current development-v6 builds to fail the tests. This PR fixes this.

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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