HBASE-7410 add snapshot/clone/restore/export docs to ref guide

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1468972 13f79535-47bb-0310-9956-ffa450edef68
HBASE-8321 Log split worker should heartbeat to avoid timeout when the hlog is under recovery

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1468974 13f79535-47bb-0310-9956-ffa450edef68