changefeedccl: improve plan rebalance algorithm

It's useful to have balanced changefeed plans, but there are trade-offs involved with setting `changefeed.default_range_distribution_strategy="balanced_simple"`. 1) Since it's a cluster setting, it applies to all changefeeds, 2) the algorithm naively redistributes spans across aggregators with no regard for replica placement. The algorithm could be improved by only rebalancing spans to aggregators mapped to replicas.

Jira issue: CRDB-51174