OAK-1338: Implement QueryStat from Jackrabbit API
Use "QueryStat" instead of "QueryStats" ofr the QueryStat MBean in order to be compatible with Jackrabbit 2

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1571967 13f79535-47bb-0310-9956-ffa450edef68