feat: enable OnOpen for connected UDP socket

[panjf2000]

Fixes #549

[codecov]

Codecov Report

Attention: Patch coverage is 96.77419% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 79.08%. Comparing base (54f81b6) to head (9cf2f2d).

Files	Patch %	Lines
eventloop_unix.go	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #554      +/-   ##
==========================================
+ Coverage   79.00%   79.08%   +0.08%     
==========================================
  Files          25       25              
  Lines        2091     2109      +18     
==========================================
+ Hits         1652     1668      +16     
- Misses        304      305       +1     
- Partials      135      136       +1     

Flag	Coverage Δ
unittests	79.08% <96.77%> (+0.08%)	⬆️

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