[RSGroup] Support renaming rsgroup

Rsgroup name once is decided at the beginning, it is difficult to rename it.

Current approach is removing all tables and servers back to default rsgroup, then delete it and add a rsgroup with the new name, after that moving regions and servers back. Or without moving back, if machine resources is ample. Anyway, it is an expensive operation: moving regions, breaking region's locality.

And given that rsgroup is one kind of managements in cluster, and management sometimes changes, renaming is necessary.

It is simple in implementation. I'm working on it.