Fix/improve X-Forwarded-For/throttling testing

[hummeltech]

• Previously it was not actually sending the header
• Now runs in serial, regardless of parallel specification
• Now runs only once, rather than for each map layer

Also:

• Removed no-longer used storage_mutex
  • Was only used for Apache HTTP Server < v2.4

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.03%. Comparing base (d67a672) to head (0d8cc51).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   78.28%   79.03%   +0.74%     
==========================================
  Files          24       24              
  Lines        5859     5847      -12     
==========================================
+ Hits         4587     4621      +34     
+ Misses       1272     1226      -46     

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