Move hbase-rsgroup code into hbase-server code

Keeping rsgroup code into separate module is causing many problem. HBASE-22740 and HBASE-24212 are blocked because of this.

In master branch hbase-rsgroup code is already moved into hbase-server. It is better to be in sync with master branch so issue fixes can be applied on branch-2 easily.

This jira moves hbase-rsgroup code into hbase-server as it is, does not make change in protobuff etc.