kv: load-based rebalancing in multi-region configurations is non-optimal

**Is your feature request related to a problem? Please describe.**
In a recent customer investigation, there were a a large number of load-based rebalancing. The system is multi-region and the leaseholders each pinned to 1 of 3 regions, but only 2 of the 3 regions are actually in use. The configuration is RF=5 with 2 replicas in each of the "primary" regions and 1 in the secondary. Each region has 6 nodes (18 nodes total). While running workloads, there is a lot of load based rebalancing in the primary regions, specifically in region 1 there are replicas transferring back and forth continually between a few nodes.

**Describe the solution you'd like**
There should not have been any load based rebalancing in this scenario.

Jira issue: CRDB-31137