Asymmetry of GET /rooms/:room_id/state vs. .../state/:type/:key (SPEC-264)

If you `GET` the entire room state, what you get back is a JSON list, containing individual events. Within those events, are content keys.

If you `GET` a single state message, what comes back is just the value of `content`, not the entire event.

When we get around to designing client API v2 for this, we should redesign it for more symmetry. The specific use-case I have in mind is wanting to find the `event_id` of my own `m.room.member` event in a room. I can't just `GET` that one event because I'll only get the content; instead I have to `GET` the entire room state and iterate over it to find the appropriate event.

(Imported from https://matrix.org/jira/browse/SPEC-264)

(Reported by @leonerd)

Start by comparing the documented GET /rooms/:room_id/state and GET /rooms/:room_id/state/:type/:key endpoints described in the issue. Define a v2 response design that makes whole-state and single-state retrieval symmetric and supports finding the event_id for an m.room.member event; the issue does not identify files or tests.

Written by the indexing model from the issue text.