NLogLoggingConfiguration - Fix leak in RegisterChangeCallback

[snakefoot]

See also: https://stackoverflow.com/questions/57485634/memory-leak-why-too-many-ichangetoken-objects

[codecov-io]

Codecov Report

Merging #322 into master will decrease coverage by 0.38%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
- Coverage   81.44%   81.05%   -0.39%     
==========================================
  Files          12       12              
  Lines        1094     1098       +4     
  Branches      191      193       +2     
==========================================
- Hits          891      890       -1     
- Misses        141      143       +2     
- Partials       62       65       +3

Impacted Files	Coverage Δ
...ensions.Logging/Config/NLogLoggingConfiguration.cs	91.86% <60%> (-2.29%)	⬇️

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