Dynamic address length calculation in shadowsocks udp encoding

[xiaokangwang]

This pull request dynamically calculate the address length in shadowsocks udp ending, see also: #3564 (comment)

[codecov]

Codecov Report

❌ Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 23.21%. Comparing base (b2c3b25) to head (e1a926f).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
proxy/shadowsocks/protocol.go	53.84%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3588   +/-   ##
=======================================
  Coverage   23.20%   23.21%           
=======================================
  Files         836      836           
  Lines       45442    45454   +12     
=======================================
+ Hits        10547    10551    +4     
- Misses      33678    33685    +7     
- Partials     1217     1218    +1     

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