Log current memory usage before & after handling each request

[jwodder]

Part of #118.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 45.77%. Comparing base (4c0005d) to head (201ed47).

Files	Patch %	Lines
src/main.rs	0.00%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #120      +/-   ##
==========================================
- Coverage   45.95%   45.77%   -0.18%     
==========================================
  Files          24       24              
  Lines        3595     3609      +14     
==========================================
  Hits         1652     1652              
- Misses       1943     1957      +14     

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