Syntactic changes plus a small refactoring.

[Link512]

• more use of auto
• moved std::function to a using Removed queue not empty condition when exiting thread because that will cause the destructor to hang until the queue is empty

[Link512]

I was just talking in terms of convenience, if the queue will not drain, it means you have to wrap every future.get() in a try/catch and have some kind of fallback mechanism if you're task was not executed. But I suppose that if you specifically choose to initialize the pool in non-draining mode, you are acknowledging the fact that you will have to try/catch everything. I will close this request and update the pull request.