refactor: Route stream key resolution to one loop

[kimsun721]

Route stream key resolution to one loop

Why

A stream key carried no loop, so resolving one asked every loop. It was the only HTTP request that fanned out: its reply needed an mpsc, it answered 503 if any loop was full, and one wrong key cost a message on every loop.

Changes

• A stream key is SK_<loop>_<random>, like a room id. The loop comes from the room id when the key is issued or reissued.
• Shards::sender_by_key reads the loop from either kind of id, replacing sender_for.
• Resolution asks that loop alone and awaits a oneshot reply.
• A key without a loop number, or with one past the last loop, gets 401 without a loop being asked.
• A loop that drops the reply now gives 500, not 401.
• ADR 0013 records the key format and supersedes the stream key resolution in ADR 0012. The architecture overview follows.

Tested

• cargo test: all suites pass.
• New test: a wrong key gets 401 whether its loop exists or not. The client has a timeout, so a request left waiting fails the test.

Closes #85