feat:splitstore:limit moving gc threads

[ZenGround0]

Related Issues

After recent fix #10392 should be fine but we just had a bug report that wasn't caught during test so let's let that change sit a little before incorporating in a release.

However we do want to add configuration that makes moving GC more common to the release for better disk management. This PR is a small subset of #10392 which should make moving gc less likely to cause out of sync that is really safe without more testing time.

Proposed Changes

Additional Info

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[arajasek]

LGTM -- so we wanna land this in master, backport it onto the release, and then rebase #10392 on top of latest master (which will have this change)?