should connect target server synchronously

[MarkLux]

the ProxyFrontendHandler#channelActive would try to connect the target server and get a channelFuture, but this code works asynchronously, therefore, when request comes, the channelRead is invoked while the outboundChannel is not ready (in my case, it's state is pending), and the proxy work failed. I rewrite the channelActive with synchronously connecting the target server, it works well.