Tool for validating model changes

[dmdashenkov]

In Protobuf, it is possible to make backwards-compatible changes to Proto definitions. In Spine, we often rely on the users making all the changes to their models backwards-compatible. However, this is often not what the users do.

In order to help users not to shoot their own feet, we should:

1. Educate. Write guidelines on how model migration works in Spine.
2. Automate. Provide a build-time check which looks for changes in the domain model. The tool could save the previous versions of the model pacakge-lock-style.