fix: sales concurrency bug

[AuHau]

Closes #532.

Since the onSale callback can potentially be overridden (for example, like in testsales), I have decided to create a new onFilled function instead. This function is not public and should only be used to clear the futures of the Queue.

Other options include making the onSale function "private" or supporting multiple functions in hooks.

[emizzle]

LGTM!
I think we can simply remove OnSale, as it's not really being used, and OnFilled is more accurate.

[AuHau]

I think we can simply remove OnSale, as it's not really being used, and OnFilled is more accurate.

The problem with replacing OnSale with OnFilled is that if OnFilled would be used by some external component (like tests for example) than it would potentially break the Sales module as currently the hooks allow only one hook to be used and if another is added than it replaces the original. And we use the OnFilled for unblocking the queue workers, so if tests would use it for some tests assertions then it would break the unblocking of the queue workers mechanism. I will add comment about it to the code and then merge it in the current form.