feat: support edge-triggered I/O

[panjf2000]

Fixes #573

[codecov]

Codecov Report

Attention: Patch coverage is 71.70418% with 88 lines in your changes are missing coverage. Please review.

Project coverage is 78.86%. Comparing base (601bfdf) to head (a0fd910).

Files	Patch %	Lines
reactor_epoll_default.go	65.78%	16 Missing and 10 partials ⚠️
reactor_kqueue_default.go	75.00%	16 Missing and 4 partials ⚠️
eventloop_unix.go	53.12%	10 Missing and 5 partials ⚠️
connection_unix.go	81.13%	8 Missing and 2 partials ⚠️
connection_linux.go	60.00%	3 Missing and 3 partials ⚠️
reactor_epoll_ultimate.go	62.50%	3 Missing ⚠️
reactor_kqueue_ultimate.go	62.50%	3 Missing ⚠️
connection_bsd.go	86.66%	2 Missing ⚠️
engine_unix.go	84.61%	0 Missing and 2 partials ⚠️
acceptor_unix.go	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #576      +/-   ##
==========================================
- Coverage   79.08%   78.86%   -0.23%     
==========================================
  Files          25       25              
  Lines        2109     2186      +77     
==========================================
+ Hits         1668     1724      +56     
- Misses        305      319      +14     
- Partials      136      143       +7     

Flag	Coverage Δ
unittests	78.86% <71.70%> (-0.23%)	⬇️

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