HBASE-7590 Add a costless notifications mechanism from master to regionservers & clients

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.95@1458188 13f79535-47bb-0310-9956-ffa450edef68
HBASE-7590 Add a costless notifications mechanism from master to regionservers & clients - new files

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.95@1458202 13f79535-47bb-0310-9956-ffa450edef68