Refactor sendfile

[asvetlov]

The main change is using add_done_callback to unregister writer early on future cancellation.

[codecov-io]

Codecov Report

Merging #3383 into master will decrease coverage by 0.05%.
The diff coverage is 96.29%.

@@            Coverage Diff            @@
##           master   #3383      +/-   ##
=========================================
- Coverage   97.96%   97.9%   -0.06%     
=========================================
  Files          44      44              
  Lines        8489    8497       +8     
  Branches     1376    1375       -1     
=========================================
+ Hits         8316    8319       +3     
- Misses         74      77       +3     
- Partials       99     101       +2

Impacted Files	Coverage Δ
aiohttp/web_fileresponse.py	94.67% <96.29%> (-2.85%)	⬇️

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