refactor(stream): make new_boxed_executor an async trait function

[stdrc]

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What's changed and what's your intention?

As simple as title.

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[BugenZhao]

Interesting. So we're actually not relying on the Future to be Send?

[stdrc]

So we're actually not relying on the Future to be Send?

It looks so, though I don't clearly know why and why we need Send before.