Create a task per request handling.

[asvetlov]

Fixes #3406

A separate task for every request doesn't produce any performance degradation on benchmarks.

Tests update is needed to wait for not only a task started by connection_made() but also a nested task.

[kxepal]

LGFM. Indeed, it doesn't affects performance (or it does, but those numbers get lost in statistical error).

[samuelcolvin]

Looks good.

We should document this and state prominently that aiohttp supports ContextVar?

Perhaps with an example of how to use it.

[asvetlov]

@samuelcolvin what docs part do you suggest?
A chapter in web_quickstart.rst maybe?

[samuelcolvin]

I guess.

Not sure if really matters, just if someone searches "aiohttp ContextVar" they should get a result in docs.

[codecov-io]

Codecov Report

Merging #3442 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3442      +/-   ##
==========================================
+ Coverage   97.93%   97.93%   +<.01%     
==========================================
  Files          44       44              
  Lines        8538     8539       +1     
  Branches     1378     1378              
==========================================
+ Hits         8362     8363       +1     
  Misses         72       72              
  Partials      104      104

Impacted Files	Coverage Δ
aiohttp/web_protocol.py	92.2% <100%> (+0.02%)	⬆️

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[asvetlov]

I've started to write a doc but found that there are questions about contextvars initialization:

• Where is safe to set up a context?
• How it works with the context modification code in Application.on_startup()?
• The PR creates a context copy for every request processing, sure. But what is the original context source?

Let me merge the PR as is and solve the questions in separate one (along with documentation update). Maybe run_app() code should be modified as well, I need to check.

[lock]

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