Less talk about "server"

Because the WebODF editing is not needing a server for collaboration, but could also do P2P e.g. with WebRTC, I propose to not use the term "server" for the host of a session in general, to prevent server-only thinking.

Not sure about the best lingo though. For a start:
* what about renaming the "server/" folder to "backends/" (or "backend/"?)?
* what about talking about "master session" when the central session object somewhere is meant which holds the one true version of all data of the session? Even in P2P there needs to be one central entity which would be meant with "the" current document.
> Should be mentioned now, agreed. No idea about a proper wording. Also what your/our long-term plans are with it? For now I see it as a demo/testing backend, especially given such twisting options like "Connect/Disconnect" and "Latency". Do you have any more plans with it?
> The code might be reusable e.g. for a P2P system with WebRTC (still hope to look into that soonish), but perhaps not directly.

At this point I have no immediate plans. I intended the code is neat enough to provide a good teaching system for how-to write a backend (it avoids a lot of the complex callback interleaving of the pullbox one). I also tried to ensure the code could be easily adapted for a WebRTC type system (hence, trying to keep the fetching process decently abstract etc.). 

But as of today, I am happy enough with that backend for my immediate goal (collab fuzzing!).