Rename PayloadWriter to StreamWriter

[pfreixes]

What do these changes do?

Rename the PayloadWriter class to StreamWriter, having consistency with the reader side which has a class called StreamReader.

Are there changes in behavior for the user?

No

Related issue number

#2109

Checklist

• I think the code is well written
• Unit tests for the changes exist

[codecov-io]

Codecov Report

Merging #2654 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2654   +/-   ##
=======================================
  Coverage   97.98%   97.98%           
=======================================
  Files          39       39           
  Lines        7245     7245           
  Branches     1257     1257           
=======================================
  Hits         7099     7099           
  Misses         45       45           
  Partials      101      101

Impacted Files	Coverage Δ
aiohttp/http.py	100% <ø> (ø)	⬆️
aiohttp/payload.py	98.74% <ø> (ø)	⬆️
aiohttp/http_writer.py	100% <100%> (ø)	⬆️
aiohttp/web_protocol.py	91.57% <100%> (ø)	⬆️
aiohttp/client_reqrep.py	97.36% <100%> (ø)	⬆️
aiohttp/web_fileresponse.py	97.87% <100%> (ø)	⬆️
aiohttp/abc.py	100% <100%> (ø)	⬆️

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[asvetlov]

Thanks!

[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