ref: Update actix integration to actix-web 4

[Swatinem]

This also includes the following drive-by changes:

• Make sure our examples are more consistent with usage of release_name/debug options.
• Simplify transaction naming / avoid intermediate String allocation.
• Update the Changelog.

supercedes #316
fixes #388
fixes #327
fixes #308

[codecov-commenter]

Codecov Report

Merging #437 (2810263) into master (871f727) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

❗ Current head 2810263 differs from pull request most recent head 28bdf6d. Consider uploading reports for the commit 28bdf6d to get more accurate results

@@            Coverage Diff             @@
##           master     #437      +/-   ##
==========================================
- Coverage   80.52%   80.51%   -0.02%     
==========================================
  Files          73       73              
  Lines        8293     8302       +9     
==========================================
+ Hits         6678     6684       +6     
- Misses       1615     1618       +3