rssStub in HRegionServer is not thread safe and should not directly be used

While working on a patch for HBASE-22060, I noticed that a unit test started failing because region server crashed with NPE during initialization and after aborting the master. It turned out that access to the rssStub was not synchronized.

The existing code in HRegionServer already takes care of this fact by copying and null checking in most places, but there are a couple ones that are not so careful. Those are in reportForDuty and abort methods.