If a source shard primary steps down during a movePrimary, it's possible that the MovePrimarySourceManager variable never gets cleared. If the same node steps up and retries to move the same database, it will cause an invariant.

We should clear the MovePrimarySourceManager variables for databases for which a shard is primary on step-up (somewhere around here. If we do so, we can guarantee a clean state for all attempted movePrimaries.