fix continous growth of app data in pooled requests

[robjtede]

PR Type

What kind of change does this PR make?

Bug Fix

PR Checklist

Check your PR fulfills the following:

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.

Overview

A change made in PR #1486 to support multiple layers of application data caused a leak of app data for each request object that was reclaimed by the request pool. This PR makes a simple change to set fresh app data for a re-used request object rather than appending to the existing vec of app data layers.

This PR also adds some documentation around the request pool so readers can better understand its purpose and life cycle.

fixes #1606
fixes #1607

[codecov-commenter]

Codecov Report

Merging #1609 into master will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1609      +/-   ##
==========================================
- Coverage   52.94%   52.90%   -0.05%     
==========================================
  Files         124      124              
  Lines       11784    11773      -11     
==========================================
- Hits         6239     6228      -11     
  Misses       5545     5545              

Impacted Files	Coverage Δ
src/app_service.rs	88.11% <100.00%> (ø)
actix-http/src/client/config.rs	88.88% <0.00%> (-11.12%)	⬇️
actix-http/src/encoding/encoder.rs	80.30% <0.00%> (-1.52%)	⬇️
actix-http/src/config.rs	58.71% <0.00%> (-0.92%)	⬇️
awc/src/sender.rs	25.89% <0.00%> (-0.90%)	⬇️
src/types/payload.rs	80.59% <0.00%> (-0.75%)	⬇️
actix-http/src/response.rs	40.80% <0.00%> (-0.34%)	⬇️
src/test.rs	88.58% <0.00%> (-0.28%)	⬇️
actix-http/src/h1/dispatcher.rs	56.53% <0.00%> (-0.26%)	⬇️
src/rmap.rs	94.15% <0.00%> (-0.04%)	⬇️
... and 8 more

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[JohnTitor]

Looks great, thanks for fixing quickly!

[attila-lin]

Nice work!
How about release a new version?