Stop response download from signal handler

[elacuesta]

Allow the cancellation of a download while it's in progress.

Motivated by #4205 (comment).

Tasks:

• Implementation
• Tests
• Docs
  • Exceptions topic
  • Request/Response topic
  • FAQ entry
  • Note in the bytes_received signal docs

[codecov]

Codecov Report

Merging #4559 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4559      +/-   ##
==========================================
+ Coverage   84.56%   84.62%   +0.05%     
==========================================
  Files         164      164              
  Lines        9931     9948      +17     
  Branches     1477     1480       +3     
==========================================
+ Hits         8398     8418      +20     
+ Misses       1266     1265       -1     
+ Partials      267      265       -2     

Impacted Files	Coverage Δ
scrapy/core/downloader/handlers/http11.py	93.03% <100.00%> (+0.30%)	⬆️
scrapy/exceptions.py	92.59% <100.00%> (+1.28%)	⬆️
scrapy/utils/signal.py	97.87% <100.00%> (+4.39%)	⬆️
scrapy/utils/trackref.py	85.71% <0.00%> (+2.85%)	⬆️

[Gallaecio]

Beautiful.

[kmike]

Thanks @elacuesta!