Snap service start ordering

@mborzecki I’ve moved the messages into this topic as it covers exactly that same aspect. There are also some related points in this other topic that is relevant here.

About the proposal and the implementation, it sounds better to name these as simply after and before, without the start- prefix, so they are implying an order without implying the fact starting the services is being requested. Otherwise it gets pretty confusing because you can order one service after two others, but that doesn’t imply starting it with these services, and once we introduce the fields starts-with and runs-with, it will get even more confusing.