server descriptor reformatting

Along the lines of !305 (merged). Example branch structure.

If this sseem like a good way of presenting things I propose to carry on roughly like this. Probably rather larger MRs than this, and in at least two series: one for server-descriptor-format and one for consensus-formats.

I think this doesn't need to block on #286 (closed), which I will pursue in parallel.

CC @nickm