This patch is related to LBCLASSIC-53

- This is a modified and hopefully improved version of Raph Goer's patch.

  MDCLevelFilter was renamed as DynamicThresholdFilter.
  
  In particular, the syntax for configuring a DynamicThresholdFilter
  is much shorter and convenient than previously. Here is an example:.
  
	<turboFilter class="ch.qos.logback.classic.turbo.DynamicThresholdFilter">
		<Key>userId</Key>
		<DefaultThreshold>ERROR</DefaultThreshold>
		<MDCValueLevelPair>
			<value>user1</value>
			<level>INFO</level>
		</MDCValueLevelPair>
		<MDCValueLevelPair>
			<value>user2</value>
			<level>TRACE</level>
		</MDCValueLevelPair>

	</turboFilter>

  See also turboDynamicThreshold.xml.

  However, I am not 100% satisfied with the semantics of
  DynamicThresholdFilter. Expect it to change according to
  feedback received on the mailing lists.