HTTP/2 support for connections to origin servers & DoH client

Closes #625.

ingress/origin_service.go

@@ -284,6 +284,7 @@ func newHTTPTransport(service OriginService, cfg OriginRequestConfig, log *zerol
 		TLSHandshakeTimeout:   cfg.TLSTimeout.Duration,
 		ExpectContinueTimeout: 1 * time.Second,
 		TLSClientConfig:       &tls.Config{RootCAs: originCertPool, InsecureSkipVerify: cfg.NoTLSVerify},
+		ForceAttemptHTTP2:     true,
 	}
 	if _, isHelloWorld := service.(*helloWorld); !isHelloWorld && cfg.OriginServerName != "" {
 		httpTransport.TLSClientConfig.ServerName = cfg.OriginServerName

tunneldns/https_upstream.go

@@ -135,6 +135,7 @@ func configureClient(hostname string, maxUpstreamConnections int) *http.Client {
 		MaxIdleConns:       1,
 		MaxConnsPerHost:    maxUpstreamConnections,
 		Proxy:              http.ProxyFromEnvironment,
+		ForceAttemptHTTP2:  true,
 	}
 	_ = http2.ConfigureTransport(transport)