Write to global variable once to prevent data races

[pscheid92]

Before:
We write to the global variable 'headerXRequestID' on every request. This leads to data races and a slight slowdown for each request.

After:
We write to the global variable 'headerXRequestID' once during the setup of the middleware.

For more details see issue #24

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.28%. Comparing base (f9bc391) to head (291403b).
Report is 31 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   94.11%   94.28%   +0.16%     
==========================================
  Files           2        2              
  Lines          34       35       +1     
==========================================
+ Hits           32       33       +1     
  Misses          2        2              

Flag	Coverage Δ
go-	94.28% <100.00%> (+0.16%)	⬆️
go-1.14	94.28% <100.00%> (+0.16%)	⬆️
go-1.15	94.28% <100.00%> (+0.16%)	⬆️
go-1.16	94.28% <100.00%> (+0.16%)	⬆️
go-1.17	94.28% <100.00%> (+0.16%)	⬆️
go-1.18	94.28% <100.00%> (+0.16%)	⬆️
go-1.19	94.28% <100.00%> (?)
macos-latest	94.28% <100.00%> (+0.16%)	⬆️
ubuntu-latest	94.28% <100.00%> (+0.16%)	⬆️

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