bug: alleviate the data race between Release() and Reboot()

[codecov[bot]]

Codecov Report

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

Project coverage is 90.54%. Comparing base (1933478) to head (d84b9f5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #330 +/- ## ========================================== - Coverage 91.02% 90.54% -0.48% ========================================== Files 11 11 Lines 802 804 +2 ========================================== - Hits 730 728 -2 - Misses 50 53 +3 - Partials 22 23 +1 ``` | [Flag](https://app.codecov.io/gh/panjf2000/ants/pull/330/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/ants/pull/330/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | `90.54% <100.00%> (-0.48%)` | :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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