Use async instead of defered for threadpool threads.

[panyx0718]

I don't have much background. Feel free to let me know "deferred" is the expected
behavior.

Reason for the change:
defered means you need to call get/wait first, then
the thread starts to run.
async means the thread starts ASAP, wait/get are only
used to get the result.

The code comment says:
To wait for the completion of the task, call std::future::wait()

Hence, user would expect wait() is not a method to start the thread.
It seems more reasonable to start running when the thread is available
instead of waiting for a wait/get call.

[panyx0718]

I printed out the starting sequence of deferred and async:
In deferred:
waiting
running
running
running
running done
running done
waiting done
waiting
waiting done
waiting
running done
waiting done
waiting all done

In async:
running
running
running
waiting
running done
running done
running done
waiting done
waiting
waiting done
waiting
waiting done
waiting all done

[paddle-bot-old]

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