Websockets refactoring

[asvetlov]

A part of #2698

No Public API change

[codecov-io]

Codecov Report

Merging #2836 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2836      +/-   ##
==========================================
- Coverage   97.97%   97.97%   -0.01%     
==========================================
  Files          39       39              
  Lines        7445     7444       -1     
  Branches     1307     1307              
==========================================
- Hits         7294     7293       -1     
  Misses         48       48              
  Partials      103      103

Impacted Files	Coverage Δ
aiohttp/http_websocket.py	98.6% <100%> (-0.01%)	⬇️

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[new issue]: https://github.com/aio-libs/aiohttp/issues/new