feat: goroutine exits immediately

[exfly]

---

name: goroutine exits immediately
about: goroutine exits immediately, lazy init defaultAntsPool
title: 'goroutine exits immediately'
labels: ''
assignees: 'panjf2000'

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

optimizations

First of all, thanks to ants for maintaining a good pool implementation. In the process of using it, it was found that some goroutines did not exit when Release was called. This mr change will cause all goroutines to exit immediately.
Except for the first time, the global defaultAntsPool will be created by default. In this mr, defaultAntsPool will be created the first time it is called.

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

TODO

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

TODO

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

TODO

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).

[panjf2000]

It seems that you're trying to make the goroutine of purgePeriodically quit once the pool.Release() is called, however, what's wrong about this goroutine quitting only after all other worker goroutines quit?