bug(cloudflared): Set the MaxIdleConnsPerHost of http.Transport to proxy-keepalive-connections

Setting the MaxIdleConns is not enough, the MaxIdleConnsPerHost must be set as well.
Otherwise, http.Transport will use the DefaultMaxIdleConnsPerHost, which is 2,
and then the connection pool will have only 2 connection hold.

cmd/cloudflared/tunnel/configuration.go

@@ -197,6 +197,7 @@ func prepareTunnelConfig(
 	httpTransport := &http.Transport{
 		Proxy:                 http.ProxyFromEnvironment,
 		MaxIdleConns:          c.Int("proxy-keepalive-connections"),
+		MaxIdleConnsPerHost:   c.Int("proxy-keepalive-connections"),
 		IdleConnTimeout:       c.Duration("proxy-keepalive-timeout"),
 		TLSHandshakeTimeout:   c.Duration("proxy-tls-timeout"),
 		ExpectContinueTimeout: 1 * time.Second,