Resource versions tracking

Related to #27

Every time a resource has changed, we can represent this change as a transition from _id1_ to _id2_. This transition we can store in replicated state (or send to another app instance, but it will complicate implementation and should be avoided if possible). Afterwards, it should ease conflicts resolution.

Start by reading related issue #27 and tracing how resource changes and replicated state are currently represented. The issue names no files, tests, or entry points, and the intended transition format and conflict-resolution behavior still need to be defined before implementation can be considered done.

Written by the indexing model from the issue text.