Use a DiscardingTaskGroup

[Joannis]

https://github.com/hummingbird-project/swift-jobs/blob/main/Sources/Jobs/JobQueueHandler.swift#L42

This needs to become a DiscardingTaskGroup, because even a Void.self job allocated memory per job

[adam-fowler]

We need to be able to wait on the results of a child task running which might make this difficult. But I was also of the understanding if I wait on the result then it is dropped and the memory is freed. I can have a look