bug: fix the missing log.Lmsgprefix in go1.13

[piaodazhu]

---

name: Pull request
about: Propose changes to the code
title: ''
labels: ''
assignees: ''

1. Are you opening this pull request for bug-fixs, optimizations or new feature?

bug-fixs

2. Please describe how these code changes achieve your intention.

暂时先在 ants.go 里自定义了一个 log.Lmsgprefix 的值 64 。

3. Please link to the relevant issues (if any).

[Bug]: go.mod 中的go version(1.13)和ants.go存在不匹配 #274

4. Which documentation changes (if any) need to be made/updated because of this PR?

ants.go

4. Checklist

• I have squashed all insignificant commits.
• I have commented my code for explaining package types, values, functions, and non-obvious lines.
• I have written unit tests and verified that all tests passes (if needed).
• I have documented feature info on the README (only when this PR is adding a new feature).
• (optional) I am willing to help maintain this change if there are issues with it later.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b32591f) 92.45% compared to head (06ba561) 92.45%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #275   +/-   ##
=======================================
  Coverage   92.45%   92.45%           
=======================================
  Files           9        9           
  Lines         769      769           
=======================================
  Hits          711      711           
  Misses         44       44           
  Partials       14       14           

Flag	Coverage Δ
unittests	92.45% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
ants.go	85.00% <ø> (ø)

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[panjf2000]

Thanks~