NLogLoggerFactory - Emulate Microsoft LoggerFactory by returning same ILogger instance

[snakefoot]

See also:

https://github.com/aspnet/Extensions/blob/03ca0e1aa41c55a94e6c04f0c1fa803986fe9e64/src/Logging/Logging/src/LoggerFactory.cs#L99

[codecov-io]

Codecov Report

Merging #380 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
+ Coverage   81.85%   82.02%   +0.16%     
==========================================
  Files          12       12              
  Lines        1163     1174      +11     
  Branches      192      193       +1     
==========================================
+ Hits          952      963      +11     
  Misses        141      141              
  Partials       70       70

Impacted Files	Coverage Δ
src/NLog.Extensions.Logging/Logging/NLogLogger.cs	80.5% <100%> (+0.21%)	⬆️
...og.Extensions.Logging/Logging/NLogLoggerFactory.cs	88.23% <100%> (+3.61%)	⬆️

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[304NotModified]

Thanks!