Add writer overflow behaviours more

[hadashiA]

#129 #131

The behavior of the WriterLoop queue when it overflows can now be configured.

Add ZLoggerOptions.FullMode,

• BackgroundBufferFullMode.Grow (default)
  • It is the same as before. Buffer as long as memory allows.
• BackgroundBufferFullMode.Wait
  • Wait until there's more room in the queue.
• BackgroundBufferFullMode.Drop
  • Drop the overflowing log entry.

( There may not yet be agreement on the name of the option.

[neuecc]

Is there a reason you didn't use WriteAsync()?
channel.Writer.WriteAsync(log).AsTask().Wait();

[hadashiA]

Fixed to use .AsTask().Wait() .

(Personally I thought that ManualResetEvent would be less costly, but we decided to use AsTask().Wait() because it is safer to leave all control to the Channel.)