switch to another gorilla/websocket package

[songbinliu]

According to the golang official statement,
This package(golang.org/x/net/websocket) currently lacks some features found in an alternative and more actively maintained WebSocket package: https://godoc.org/github.com/gorilla/websocket

The drawbacks of x/net/websocket are:

• Less actively maintained;
• Lack some features: especially the ping-pong protocol(RFC 6455), which is essential for keeping alive check.