v0.12.310

/v1/messages streams, and a stalled response no longer looks like success

If an upstream paused for more than 30 seconds during a /v1/messages response, the
client got a 200 with an empty body and no error, on a call that may have been billed.
The route also held back the whole stream until it ended.

• Live streaming: stream: true on /v1/messages now delivers events as they're
  produced.
• Longer pauses: the call fails only after 5 minutes with no data.
• Honest failures: a read that fails breaks the connection, so a cut-off answer can't
  look complete. If the client disconnects, the upstream call is cancelled.

Speech and the partner and data routes keep their 30-second limit, but a read that fails
now answers 502 instead of an empty 200.

Cache bypass does what it says

• A request sent with Cache-Control: no-cache no longer has its response stored. It used
  to skip only the lookup, so the next identical request was answered from that response.
  Thanks @0xcfloki (#556).
• Cache-Control: no-store now bypasses the response cache too. Its response used to be
  stored for 10 minutes.
• Directive names now match in any case, so NO-CACHE works.

The 30-second dedup cache still replays an identical retry, whatever the header says.

Behaviour change: these requests no longer refresh the cache, so an earlier entry stays
until it expires.

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