OAK-759 MongoMK: read from a secondary member when possible (allow tests to use replica sets; a shell script to start a replica set with 3 nodes; a first manual failover test)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1560611 13f79535-47bb-0310-9956-ffa450edef68