The LoggingEventPreSerializationTransformer is completely stateless and
therefore inherently thread-safe.  Therefore, there is no reason to
create a distinct instance for each appender.  Moreover, once created it
never needs to be modified, therefore it should be properly marked as
immutable via the final keyword.

Introducing AbstractDiscriminator base class

add ResourceExistsPropertyDefiner

Fix for LOGBACK-230 - Support attribute "optional" in include element

fix logback-767

AppenderTracker is now extends AbstractComponentTracker, removed support for sifting in Gaffer, tests pass

 StatusListener has two instances OnConsoleStatusListener and instance by
 seted classname.
 The second be Judged that is ContextAware and LifeCycle's implement and start it.
 but The first is not to Judged.

 all StatusListener must be Judged and start it.

Was protected now package-private; added javadoc comment describing
purpose and intended use.

Receiver tracking support merged.