backport-2.1: storage: make load-based replica rebalancing decisions at the store level

[jseldess]

PR: cockroachdb/cockroach#29663

From release notes:

Range replicas are now automatically rebalanced throughout the cluster to even out the amount of QPS being handled by each node. [#29663][#29663] {% comment %}doc{% endcomment %}

The experimental, non-recommended kv.allocator.stat_based_rebalancing.enabled and kv.allocator.stat_rebalance_threshold cluster settings have been replaced by an improved approach to load-based rebalancing that can be controlled via the new kv.allocator.load_based_rebalancing cluster setting. By default, leases will be rebalanced within a cluster to achieve better QPS balance. [#29663][#29663] {% comment %}doc{% endcomment %}

[rmloveland]

This was handled by PR #3921