purgePeriodically function is left running for a while after pool.Release is called

[ShivanshVij]

Describe the bug
When the Pool.Release() function is called, it can leave the purgePeriodically function running until its heartbeat time.Ticker triggers, at which point the condition on whether the pool is closed is checked by the purgePeriodically for loop.

The issue is that one cannot assume the goroutines associated with an ant pool are completely cleaned up until the purgePeriodically function is completed - and as such, tools like goleak (https://github.com/uber-go/goleak) will report a leaked goroutine in tests.

To Reproduce
Steps to reproduce the behavior:

1. Clone https://github.com/loopholelabs/frisbee
2. Check out the rpc-async branch
3. Run go test ./...
4. See error

Expected behavior
The purgePeriodically goroutine is closed when the Release function returns.

Error messages/Trace logs

goleak: Errors on successful test run: found unexpected goroutines:
[Goroutine 4 in state chan receive, with github.com/panjf2000/ants/v2.(*Pool).purgePeriodically on top of the stack:
goroutine 4 [chan receive]:
github.com/panjf2000/ants/v2.(*Pool).purgePeriodically(0x14000140000)
        /Users/shivanshvij/go/pkg/mod/github.com/panjf2000/ants/v2@v2.4.7/pool.go:69 +0x80
created by github.com/panjf2000/ants/v2.NewPool
        /Users/shivanshvij/go/pkg/mod/github.com/panjf2000/ants/v2@v2.4.7/pool.go:137 +0x380
]
FAIL    github.com/loopholelabs/frisbee 10.624s

System info (please complete the following information):

• OS: macOS
• Go Version: 1.17.6
• ants version: 2.4.7

Additional context
Add any other context about the problem here.