MongoDB implementation of Pub/Sub Service

To avoid depending on Redis, we need a MongoDB-based implementation of the Pub/Sub Service API.

References for implementation:

• http://blog.pythonisito.com/2013/04/mongodb-pubsub-with-capped-collections.html
• http://tuttlem.github.io/2014/06/14/using-tailable-cursors-on-the-mongodb-oplog-for-realtime-changes.html
• https://stackoverflow.com/questions/26592542/how-do-i-move-a-tailable-cursor-with-awaitdata-to-the-end-so-i-just-get-new-upda/26592710
• https://groups.google.com/forum/#!topic/mongodb-user/E7BSv624nBg

For MongoDB 3.6, the way to go seems to be with Change Streams:

• https://docs.mongodb.com/master/changeStreams/
• http://mongodb.github.io/mongo-java-driver/3.6/driver/tutorials/change-streams/