opt: use worker pool for asynchronous tasks on Windows

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 78.75%. Comparing base (2f1bf9b) to head (5f2d2a4). Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
connection_windows.go	55.00%	8 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #657 +/- ## ========================================== - Coverage 78.82% 78.75% -0.08% ========================================== Files 21 21 Lines 2546 2551 +5 ========================================== + Hits 2007 2009 +2 - Misses 420 422 +2 - Partials 119 120 +1 ``` | [Flag](https://app.codecov.io/gh/panjf2000/gnet/pull/657/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/657/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | `78.75% <55.00%> (-0.08%)` | :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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