opt: clamp the event-loops in ET mode to avaoid starving

[panjf2000]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 79.14%. Comparing base (4c3b84f) to head (06f0fcd).

Files	Patch %	Lines
eventloop_unix.go	52.94%	6 Missing and 2 partials ⚠️
connection_linux.go	50.00%	1 Missing and 1 partial ⚠️
connection_bsd.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #599      +/-   ##
==========================================
- Coverage   79.64%   79.14%   -0.51%     
==========================================
  Files          21       21              
  Lines        2098     2110      +12     
==========================================
- Hits         1671     1670       -1     
- Misses        294      305      +11     
- Partials      133      135       +2     

Flag	Coverage Δ
unittests	79.14% <50.00%> (-0.51%)	⬇️

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