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.

LOGBACK-246

This eliminates duplicate life cycle management code in ContextBase and
LogbackValve.

This essentially mirrors the implementation in ContextBase.

The context will manage the life cycle of the receiver.