Support versioning in abacus-mongoclient

[hsiliev]

Currently we scale microservices as applications (aggregator-0, aggregator-1, etc.). Each application receives submissions for particular organization (see #536). To guarantee consistency we use locks.

This means that if we scale application instances (aggregator-0 has 2 instances) these locks will no longer function correctly. For CouchDB we can prevent modification of docs with bad revision.

To have the same behaviour we need to version documents stored in Mongo and simulate the Couch revisions.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/140011049

The labels on this github issue will be updated when the story is started.