AutoFlushTargetWrapper - Added AsyncFlush property

[snakefoot]

Allow the following setup to work out of the box:

  <target name="mailautoflush" xsi:type="AutoFlushWrapper" condition="level >= LogLevel.Error">
    <target name="mailbuffer" xsi:type="BufferingWrapper" bufferSize="1000000">
      <target name="mail" xsi:type="Mail" ... />
    </target>
  </target>

Before the BufferingWrapper would capture the LogEvent, so it wouldn't reach the final target, so the attached flush-operation was not executed. Disabling AsyncFlush will make AutoFlushWrapper work together with BufferingWrapper.

[codecov-io]

Codecov Report

Merging #2042 into master will decrease coverage by <1%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #2042    +/-   ##
=======================================
- Coverage      82%     81%   -<1%     
=======================================
  Files         286     286            
  Lines       19663   19678    +15     
  Branches     2309    2312     +3     
=======================================
- Hits        16042   16016    -26     
- Misses       3056    3096    +40     
- Partials      565     566     +1

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[304NotModified]

Good catch!

[304NotModified]

added to https://github.com/NLog/NLog/wiki/AutoFlushWrapper-target