refactor(quinn): Remove some usage of execute_poll

[gretchenfrage]

This commit makes there be fewer places where SendStream::execute_poll is called directly. AsyncWrite implementations now go through poll_write, and poll_write now goes through the write future. This means that execute_poll is now only called directly from write and write_chunks.

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Note: Cherry-picked from #2231.

[djc]

Why is this better?

[gretchenfrage]

Why is this better?

It's simpler. Consider that the previous versions of these lines involve a closure, whereas the updated versions don't. The previous versions of the lines involve calling two of our custom methods (execute_poll and write), whereas the updated versions only call one (poll_write or write).

Previously, the logic of integrating execute_poll and write in an appropriate way existed at 4 duplicate sites, whereas with this PR it exists only in one site (write) and the other places becomes derivative of that (for example, in #2231 this makes it simpler to replace execute_poll entirely. But I'm only bringing up that draft PR for illustratory purposes--this refactor should stand on its own.)

[Ralith]

Nice to know that poll_write isn't actually primitive, though it may make sense to keep as a discoverable convenience.