feat: support configurable I/O chunk to drain at a time in edge-triggered mode

[panjf2000]

Fixes #643

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.57%. Comparing base (ab70394) to head (5f70f7b).
Report is 2 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #646      +/-   ##
==========================================
- Coverage   80.34%   78.57%   -1.77%     
==========================================
  Files          21       21              
  Lines        2096     2535     +439     
==========================================
+ Hits         1684     1992     +308     
- Misses        285      417     +132     
+ Partials      127      126       -1     

Flag	Coverage Δ
unittests	78.57% <100.00%> (-1.77%)	⬇️

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