Async writer

[asvetlov]

Work on #2698

The change is required for #2767 too

[codecov-io]

Codecov Report

Merging #2774 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2774      +/-   ##
==========================================
- Coverage   97.98%   97.98%   -0.01%     
==========================================
  Files          39       39              
  Lines        7340     7338       -2     
  Branches     1285     1285              
==========================================
- Hits         7192     7190       -2     
  Misses         47       47              
  Partials      101      101

Impacted Files	Coverage Δ
aiohttp/client_reqrep.py	97.39% <100%> (ø)	⬆️
aiohttp/web_urldispatcher.py	99.19% <100%> (ø)	⬆️
aiohttp/multipart.py	95.47% <100%> (ø)	⬆️
aiohttp/http_writer.py	100% <100%> (ø)	⬆️

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[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