Add version structures for better handling of versions

[ionut-arm]

Service/protocol versions are used in request/response headers and in the ping operation.

At the moment these values are stored as two separate integers. It would be helpful if:

• They were represented by a Version object
• We had an explicit agreement on how to handle a difference between the version of a backend handler and the version specified in a request header handled by it.

[hug-dev]

Please see #105 for more information on versioning.

[hug-dev]

The second point of this issue has been adressed by #105. Renaming it "Add a Version objects" to handle the replacement of our minor/major/rev versions by those richer structures.

[ionut-arm]

Part of this was solved by parallaxsecond/parsec-interface-rs#41 . But we do need to figure out the bigger picture of handling different wire protocol versions within the client and the service.

[hug-dev]

The big picture of version handling should be adressed by #169