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

[panjf2000]

Fixes #643

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/panjf2000/gnet/pull/646/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/panjf2000/gnet/pull/646/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | `78.57% <100.00%> (-1.77%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan#carryforward-flags-in-the-pull-request-comment) to find out more.

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