Support region migration without specifying the region_id

[zyy17]

What type of enhancement is this?

API improvement

What does the enhancement do?

The current SQL admin API is:

migrate_region(region_id, from_peer, to_peer, [timeout]

When we want to execute migration between datanodes, we have to query all the region_id from information_schema and trigger the migrate_region one by one. It's not a practice API design.

We can wrap another API internally, which is:

migrate_regions(from_peer, to_peer, [timeout]

It doesn't need significant changes for the original implementation of region migration and provides the new operator-friendly API.

Implementation challenges

No response

[fengjiachun]

PTAL @WenyXu

[killme2008]

I take this issue, it's easy to implement it.