LogFactory Flush without threads and introduce FlushAsync

[snakefoot]

Make LogFactory.Flush() synchrone without spinning up threads. Instead provide Task FlushAsync() if wanting parallel target-flush capability.

Allow NLog to work out-of-the-box on platforms with limited thread-support. See also #4266

[snakefoot]

Could be interesting to implement IAsyncDisposable.DisposeAsync so one could do this without explicit call to LogFactory.Shutdown():

await using var nlogFactory = NLog.LogManager.Setup().LogFactory;

Then one could avoid try-finally and still get parallel flushing. Sadly enough this requires NET6 and not supported by NET48 / NetStandard2.