HBASE-8539 Double(or tripple ...) ZooKeeper listeners of the same type when Master recovers from ZK SessionExpiredException

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1482671 13f79535-47bb-0310-9956-ffa450edef68
HBASE-8508 improve unit-test coverage of package org.apache.hadoop.hbase.metrics.file (Ivan A. Veselovsky)

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1482687 13f79535-47bb-0310-9956-ffa450edef68
HBASE-8547 Fix java.lang.RuntimeException: Cached an already cached block

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1482704 13f79535-47bb-0310-9956-ffa450edef68