NLogLoggingConfiguration - Fix memory-leak when using AutoReload

[snakefoot]

Trying to resolve #759 by avoiding double-registration on the config-changed-event.

• NLog v5.3.4 - https://github.com/NLog/NLog/releases/tag/v5.3.4

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.53%. Comparing base (c2875ea) to head (d605cbb).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...ensions.Logging/Config/NLogLoggingConfiguration.cs	92.30%	0 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #761      +/-   ##
==========================================
+ Coverage   81.39%   81.53%   +0.14%     
==========================================
  Files          19       19              
  Lines        1806     1809       +3     
  Branches      322      322              
==========================================
+ Hits         1470     1475       +5     
+ Misses        192      190       -2     
  Partials      144      144              

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[snakefoot]

Hooking into FileNamesToWatch and ConfigurationChanged to enable AutoReload feels like a hack.

Think FileNamesToWatch-property should be obsoleted, and removed with NLog v6. Have now made initial step here: NLog/NLog#5577