Previously, ctx->out was not cleared in ngx_http_proxy_v2_reinit_request(),
which could cause stale HTTP/2 control frames (SETTINGS ACK, PING ACK,
WINDOW_UPDATE) queued for the old upstream connection to be sent to a new
upstream connection during a retry.

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

If a buffered request body wasn't fully sent, such as on early upstream
response or limited by flow control, unsent buffers could remain in the
input or busy chains when switching to the next upstream server.  This
resulted either in the invalid request sent or a stalled connection.

The fix is to reset chains similar to ngx_http_upstream_reinit().

The module allows to use HTTP/2 protocol for proxying.
HTTP/2 proxying is enabled by specifying "proxy_http_version 2".

Example:

    server {
        listen 8000;

        location / {
            proxy_http_version 2;
            proxy_pass https://127.0.0.1:8443;
        }
    }

    server {
        listen 8443 ssl;
        http2 on;

        ssl_certificate certs/example.com.crt;
        ssl_certificate_key certs/example.com.key;

        location / {
            return 200 foo;
        }
    }