feat: implement pool.Waiting() API

Fixes panjf2000#157

Add a new use case

Update READMEs

Improve the exponential backoff algorithm in spin-lock

go mod tidy

Fix an issue that blocks all waiting callers when all workers panic

Fixes panjf2000#146 panjf2000#147

fix: Memory leak (panjf2000#114)

Fixes panjf2000#113

Avoid memory leak (panjf2000#107)

Improve the unlimited pool

Updates panjf2000#90

Renew go version