NLogLogger - Faster checking if LogLevel IsEnabled

[snakefoot]

When using NLogProviderOptions.RemoveLoggerFactoryFilter = true, then Microsoft LoggerFactory will arrive with all LogEvents on all LogLevels (No longer "protected" by Microsoft LoggerFactory own Filtering-logic)

Reduced to single switch from Microsoft LogLevel to NLog Logger-IsEnabled. For faster discarding of Debug / Trace-events.

Instead of making 2 switches:

• Microsoft LogLevel -> NLog LogLevel
• NLog LogLevel -> NLog Logger-IsEnabled.