Refactor coprocessor

## Enhancement

I'm making some changes to the coprocessor recently and found that the code of features is tightly coupled. This issue intends to make the code of the coprocessor clear from the following perspective.

- Separate the iterator build path of TiFlash, TiKV, and TiDB
- It needs to iterate over the regions/buckets when building tasks, making it expandable
- The result of OOM's test is fragile when the code is changed
- Increase the test coverage of the coprocessor, it opens goroutines for concurrency but no tests can guarantee the goroutines works as expected