This also adds a previously-undocumented endpoint: /state_ids
Backfill is technically not part of this section, however it is being left untouched to make the merge with #1469 easier (which moves it out of the file).
Reference material:
* Some calls to synapse on these endpoints with a relatively simple private room.
There's two kinds of transactions currently: one with EDUs and one without. The one with EDUs is only used on /send, however the schema is still somewhat worth splitting out for simplicity.
The examples are brought apart to make them slightly more reusable for when they get dumped into the relevant sections of the spec (see TODO in server_server_api.rst)
Further, the Transactions stuff introduces tuples to the spec. The units.py has been updated to support this.
Travis Ralston