storage: fix dyncfg implementation

[rjobanp]

Motivation

• This PR fixes a previously unreported bug.

In #26186 I found that the dyncfg implementation for storage was broken and ALTER SYSTEM SET <flag> <val> commands would not work since the dyncfg option field in StorageParameters was mistakenly 'taken' before being propagated through downstream storage configuration update commands.

This is a cherry-pick of the first commit in that PR to make the fix in for the release. The tests in that PR confirm the fix.

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• 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:

[guswynn]

yikes

I tested before I changed this based on some code review