cdc: Improve scheduler

[pingyu]

Feature Request

Is your feature request related to a problem? Please describe:

Goals:

1. Ensure correctness. No KeySpan should be overlapped.
2. Be effective with many regions. One KeySpan for one region may not be suitable.
3. Reduce cost of rebalance. Un-subscribe & subscribe regions will be expensive as incremental scan is needed.
4. Boundary of KeySpan should try to align with boundary of regions, to reduce useless entry replications.

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[github-actions]

This issue is stale because it has been open 30 days with no activity.

[github-actions]

This issue is stale because it has been open 30 days with no activity.