HBASE-10306 Backport HBASE-6820 to 0.94, MiniZookeeperCluster should ensure that ZKDatabase is closed upon shutdown() (chendihao)

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1557009 13f79535-47bb-0310-9956-ffa450edef68