opt: reduce duplicate code for I/O reactors

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 79.50%. Comparing base (5cca785) to head (a5bc95b).

Files	Patch %	Lines
reactor_default.go	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #590 +/- ## ========================================== - Coverage 79.76% 79.50% -0.26% ========================================== Files 23 21 -2 Lines 2184 2098 -86 ========================================== - Hits 1742 1668 -74 + Misses 302 296 -6 + Partials 140 134 -6 ``` | [Flag](https://app.codecov.io/gh/panjf2000/gnet/pull/590/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/590/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | `79.50% <50.00%> (-0.26%)` | :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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