title: NLog File target and keepFileOpen flag
body: <p>A FileTarget object in NLog has a property (bool) called "keepFileOpen".
  By default this property is set to false, meaning that each log event will open the file, write in the file and close the file.</p>
  
  <p>The hit in performance is huge, so I specified "keepFileOpen" to true, instead of false, meaning that the file will be opened only once.</p>
  
  <p>Does anybody has an idea (or just know) why this property is by default set to "false", incuring a HUGE performance hit on logging ?</p>
  
  <p>Is there any scenarios where setting this property to true can cause issues (which would therefore make sense to default it to "false").</p>
  
  <p>Thanks !</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>Performance measurements for basic layout, writing 100K events to the logger :</p>
  
  <ul>
  <li>keepFileOpen = false (default) : ~ 101 sec</li>
  <li>keepFileOpen = true : ~ 1 sec</li>
  </ul>