Set MaxIdleConnsPerHost alongside MaxIdleConns (#3592)

Otherwise it defaults to 2, and Go will close extra connections as
soon as a request is finished.
See https://github.com/golang/go/issues/13801

util/httputil/client.go

@@ -43,6 +43,7 @@ func NewClientFromConfig(cfg config.HTTPClientConfig, name string) (*http.Client
 	var rt http.RoundTripper = &http.Transport{
 		Proxy:               http.ProxyURL(cfg.ProxyURL.URL),
 		MaxIdleConns:        20000,
+		MaxIdleConnsPerHost: 1000, // see https://github.com/golang/go/issues/13801
 		DisableKeepAlives:   false,
 		TLSClientConfig:     tlsConfig,
 		DisableCompression:  true,