Move user-triggered region admin methods (compact/split/flush) to go directly to RS

Currently region admin commands like compact, split, and flush go via HBaseAdmin from clients to the master. The master than piggybacks the requests onto the heartbeat responses.

This issue is about moving these commands to go directly from client to RS. At the same time, we should be able to make both sync and async versions (currently this is only async and there is no way to make it sync).