storage/mysql: fix bad boolean logic causing unnecessary enforcement of 'replica_preserve_commit_order' config

[rjobanp]

Motivation

• This PR fixes a previously unreported bug.

I wrote a bug 😮‍💨 -- this is causing the is_multi_threaded flag to always be set to true which is enforcing the replica_preserve_commit_order even when unnecessary. See the docstring on this method for context on this setting.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes: