bug(cloudflared): Set the MaxIdleConnsPerHost of http.Transport to proxy-keepalive-connections (#155)

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,