dial queue: fix possible goroutine leak

[raulk]

On context cancellation, the ChanQueue stops its loop but keeps the EnqChan channel open. Under the right conditions, this could cause a worker goroutine to leak by blocking forever when sending to EnqChan, as nobody would be receiving any longer.

This race is fixed by attaching a separate context to the ChanQueue whose cancellation we control. Only after all workers have yielded (fenced by a WaitGroup), do we close the ChanQueue.

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EDIT: force pushed a simpler solution as suggested by @Stebalien: #262 (review)

[Stebalien]

This works but wouldn't a simpler fix be to select on the context when writing to EnqChan in worker()?

[anacrolix]

I think the dial queue code is too large and complex, I defer to @Stebalien if he's looked into it.