Ability configure idle timeout.

[Wondertan]

Currently, idle timeout is hardcoded, but I encountered a use case where it will be useful to change it to a higher number. Any chance you can accept a PR for this @gammazero?

[jamestrandung]

@Wondertan if you're on Go 1.18, you can use my worker pool. I retrofitted gammazero's implementation to match other features in my lib and I do provide a config to change idle timeout. https://github.com/jamestrandung/go-concurrency/tree/master/async#worker-pool