OAK-641: Improved benchmark tooling

MongoMK takes over the MongoConnection, so use a separate one for the cleanup.
Unify MongoDB java driver versions.
Use dependency management also for Guava.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1448016 13f79535-47bb-0310-9956-ffa450edef68