ExecutorService.shutdownNow returns the list of jobs that never
executed, not the list of jobs that didn't terminate.

Mutable fields in SocketAppenderBase were previously exposed as
protected fields and modified by constructors in concrete subclasses.
This commit improves encapsulation of the base class by exposing
protected constructors for subclasses to invoke rather than directly
exposing the fields.  These new constructors are marked as deprecated,
as the convenience constructors on the base classes (which are also
deprecated) are the sole reason for their existence.  A future release
should remove all but the no-arg constructor from the base class and
subclasses.

These test classes were really testing SocketAppenderBase not
SocketAppender, per se.  The new implementation of SocketAppenderBase is
significantly different and will require a new unit test suite.

Previously, the executor service used here was not consistent with a
recent change to ContextBase (to allow more than one concurrent
asynchronous task).  This commit also ensures that the executor service
is shut down when the valve is stopped.

Imports of Jetty and Tomcat packages are optional as you normally can not satisify both (typically there is either Tomcat or Jetty present as OSGi web container).

Mutable fields in SocketAppenderBase were previously exposed as
protected fields and modified by constructors in concrete subclasses.
This commit improves encapsulation of the base class by exposing
protected constructors for subclasses to invoke rather than directly
exposing the fields.  These new constructors are marked as deprecated,
as the convenience constructors on the base classes (which are also
deprecated) are the sole reason for their existence.  A future release
should remove all but the no-arg constructor from the base class and
subclasses.

These test classes were really testing SocketAppenderBase not
SocketAppender, per se.  The new implementation of SocketAppenderBase is
significantly different and will require a new unit test suite.

For consistency with logback-core.

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

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

This essentially mirrors the implementation in ContextBase.

SSLServerSocketAppender has an 'ssl' property of type SSLConfiguration.

Introducing AbstractDiscriminator base class to avoid repeated start()/stop() implementation in each concrete Discriminator

This commit adds the new appender and modifies the JoranConfigurator
to include rules for SSL configuration types.