distribution skew of primary election in keyspace group

## Enhancement Task
We have a very important goal in developing the keyspace group, which is to save resources and control the failure radius of a single TSO. Each KG independently selects its primary TSO. In the current implementation, the distribution of KGs is uniform (handled by the balancer), but the primary election functionality still relies on lock contention among TSOs. As a result, the distribution of primaries between two TSO servers can be quite random. In extreme cases, such as during a TSO server upgrade, it's possible for all primaries to end up on a single TSO server. This would significantly diminish the effectiveness of the keyspace group.