Customer review updates first implementation

[fazo96]

I wrote a first implementation of the customer review updates handling using OrbitDB.

What's missing until it can be used:

• a way to register a callback to listen for review updates from outside the API (this is already in the implementation doc)
• a format for the data in the OrbitDB feed. At the moment it just writes whatever object is given. OrbitDB can store any JSON-serializable object
• key import/export for the customer

There is also a test that writes a review updates then checks that it gets replicated automatically to a service node.

This PR includes commits from #1