nghttpx: Fix crash with --backend-http-proxy-uri option

Use NGHTTP2_REFUSED_STREAM for streams which are closed by GOAWAY

The error code NGHTTP2_REFUSED_STREAM is passed to
nghttp2_on_stream_close callback when a stream is closed because its
stream ID is strictly larger than incoming or outgoing GOAWAY.

Fix doc

nghttpx: Add affinity-cookie-secure parameter to backend option

Error callback2

nghttp2_error_callback2 is an extended version of the existing
nghttp2_error_callback by adding error code parameter.  This
deprecates nghttp2_error_callback.

Nghttpx add client serial

Add tls_client_issuer_name log variable and expose it to mruby

integration: Fix issues reported by the `go vet` tool.

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

nghttpx: Cookie based session affinity