docs(socketio): update demo & solve problem on chrome

[thonatos]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

Description of change

• update demo
• add demo-egg-socket.io
• add proxy_bind for nginx
• 关于官方socket.io文档的纠错 #2338
• egg-socket.io 丢失客户端连接时的错误信息 #1848

[codecov]

Codecov Report

Merging #2354 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2354   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          29       29           
  Lines         773      773           
=======================================
  Hits          770      770           
  Misses          3        3

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