asyncio: Don't wait for the stream to close

[florimondmanca]

Fixes #634

Basically reverts #494.

Opening this as a POC - not sure this is the best way to fix things, or that this won't break other stuff.

[florimondmanca]

Note that #494 was originally added when we had still had #369 in. But in the meantime, the asyncio.Stream interface which #369 aimed at using to prevent DeprecationWarnings was removed from Python 3.8, and #369 was reverted via #423.

So in effect most discussions in #494 about whether to stream_writer.close() are outdated.

This is why I think the best way to go for us is to do as we did before #369: just stream_writer.close(), without waiting on the stream to actually close.

(From the asyncio code it seems this schedule the closing of the stream in the background - via a future - and I guess that this future will be closed by asyncio on program exit anyway, so we should be safe.)

[gdhameeja]

Just a question.
Should we be removing call to stream_closed()? The docs suggest using stream_closed along with close()
https://docs.python.org/3/library/asyncio-stream.html#asyncio.StreamWriter.close

[tomchristie]

I don't think we neccessarily need such a long-form comment, but definately a yes on this one.