Proxy: fixed HTTP/2 upstream with caching enabled.

Proxy: fixed HTTP/2 upstream with caching enabled.

Previously, when proxy_cache and keepalive were both enabled with an
HTTP/2 upstream, the second request for a cached resource could fail with
"upstream sent frame for unknown stream" error followed by "cache file
contains invalid header".

This happened because ctx->id was set to 1 in the case when no upstream
connection exists (e.g. cache hit), making the stream id check fail when
the cached response contained frames from a different stream.

The fix is to set ctx->id to 0 when there is no upstream connection,
indicating that no real stream exists, and skip the stream id validation
in this case.  Also, ctx->id = 1 is now set only for new connections,
not in the shared done label.

Closes: https://github.com/nginx/nginx/issues/1101