Support async iterators for data uploading on client

[asvetlov]

Generators look like much more native solution over @aiohttp.streamer decorator.
Python 3.6 supports it out-of-the-box, on Python 3.5 users could install async-generator library.
@streamer should be deprecated.

[asvetlov]

aiobotocore is broken, I think publishing the feature as part of bugfix release is possible, nothing should be broken again.

[asvetlov]

See #2557 discussion for StreamReader dropping motivation

[asvetlov]

Async iterators are done by #2873

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new