feat: add concurrent option to utube

[aleclarson]

Inspired by #15, this commit adds a concurrent: number option to the utube queue type.

Additionally, you can now do :take({ utube = '...' }) to take from a specific sub-queue.

The default behavior of utube has not changed.
Only one task per utube sub-queue may run at any time.

Set concurrent to 2 and only two tasks per sub-queue may run at any time.
Set concurrent to 1/0 or math.huge and no throttling is enforced.

[aleclarson]

Alright, ready for a code review.

The is_throttled method uses the most efficient means of determining concurrency for the given sub-queue name.

When :take is called without a specific sub-queue name, we traverse all READY tasks, but make sure to avoid calling is_throttled for the same sub-queue more than once. I removed the GE iterator, because it didn't look like it was necessary. Correct me if I'm wrong there.

Edit: Actually, the is_throttled method could be changed to iterate until self.concurrent is reached, instead of using :count to count every single TAKEN task. This is only an issue if you set concurrent to anything other than 1 or 1/0 (or you leave it nil).

[LeonidVas]

Hi. Thank you for the patch. Sorry for the big latency. Please, could you to describe few use case of the feature?

[aleclarson]

It's been a while, but I guess being able to :take multiple tasks from a sub-queue at one time had a good enough use case for me to implement it. :)

Is there another way to achieve that already?

[LeonidVas]

As work around: "You can use some manager around fifo/fifottl".
I agree that this may be a good ability, but without understanding the purpose of the ability, it is useless.

[aleclarson]

To me, the purpose is obviously useful. There are times where you want to process multiple tasks from the same sub-queue at once. You can't do that without this PR. Not sure what more could be explained here. ¯\(ツ)/¯

[LeonidVas]

After some discussion, we decided not add this feature to the queue. If the situation change, we get back to the PR.