Proxy: fixed HTTP/2 upstream with revalidated cache send

Proxy: fixed HTTP/2 upstream with revalidated cache send

Previously, a revalidated cached response could fail on a cached
keepalive connection with "upstream sent frame for unknown stream"
error followed by "cache file contains invalid header".

This happened because after a 304 response, nginx parsed the cached
response while the upstream peer connection was still attached to the
request.  The HTTP/2 proxy code treated cached frames as frames from
that live connection, and could assign a real stream id instead of
treating the cached response as having no real stream.

The fix is to treat cached response parsing as cache-only regardless
of the current upstream peer connection: set the stream id to 0 and
skip live upstream control-frame handling while r->cached is set.

Closes: https://github.com/nginx/nginx/issues/1318