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

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.

Context now provides a method that is used to register a component
whose life cycle is to be managed by the context.  The implementation
in ContextBase simply manages a collection of LifeCycle components,
starting the component (if necessary) before it is added to the collection,
and stopping each registered component whenever the context is reset.

LOGBACK-833 Fixes for variables in sifting appender