opt: eliminate the inuse eventloop.cache for idle connections

[panjf2000]

Fixes #659

[codecov]

Codecov Report

Attention: Patch coverage is 76.47059% with 8 lines in your changes missing coverage. Please review.

Project coverage is 79.58%. Comparing base (db2fad2) to head (e2b890f).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
connection_unix.go	76.47%	4 Missing ⚠️
connection_windows.go	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #660      +/-   ##
==========================================
+ Coverage   78.75%   79.58%   +0.82%     
==========================================
  Files          21       21              
  Lines        2551     2537      -14     
==========================================
+ Hits         2009     2019      +10     
+ Misses        422      398      -24     
  Partials      120      120              

Flag	Coverage Δ
unittests	79.58% <76.47%> (+0.82%)	⬆️

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