Define compatibility of different master/worker/client versions

As debusine is in active development changes affecting compatibility across different machines happen. When the versions of master, worker and client do not match up, failures in communication may result.

We should define what kind of combinations are considered supported and what happens in an unsupported combination. Probably, the allowed version difference between client and master should be bigger, because we do not want to force users to upgrade their client very often. The allowed version difference between master and worker may be smaller. If they mismatch, we may consider having them not talk to each other or degrade their interaction in some way. Ideally, a version mismatch does not cause jobs to produce bad output. A policy on version compatibility also is something that may change over time. We may define that worker and master version must match for now (to ease rapid development) and relax this later. Some versioning in data structures already is being done and allows for some compatibility. We should document what we consider supported.