Fix and improve file cache control header calculation

[ChihweiLHBird]

1. Microseconds in datetime should be removed for HTTP protocol.
2. Get last_modified from async file operation instead of the non-async pathlib function.

[codecov]

Codecov Report

Merging #2486 (de9fb31) into main (0e1bf89) will increase coverage by 0.002%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##              main     #2486       +/-   ##
=============================================
+ Coverage   87.269%   87.272%   +0.002%     
=============================================
  Files           60        60               
  Lines         5098      5099        +1     
  Branches       913       913               
=============================================
+ Hits          4449      4450        +1     
  Misses         476       476               
  Partials       173       173               

Impacted Files	Coverage Δ
sanic/response.py	89.285% <100.000%> (+0.054%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0e1bf89...de9fb31. Read the comment docs.