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.

Alternative approach for LOGBACK-849

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.

Since ContextBase has never before implemented LifeCycle, there are lots
of existing uses (in tests) that would need to be modified if calling
the LifeCycle methods is made a strict requirement.  By using a lazy
initialization of executor service, we can allow existing uses to ignore
the lifecycle methods, and focus our attention on those few cases where
it calling the stop method really matters.

This will allow logback-core and logback-access to both use the same
executor configuration and shutdown logic.

LOGBACK-300: Fixed OSGi-Manifest

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).

The JSSE defines several system properties that are used to create key
store and trust store objects for the system default SSL context.  This
commit allows these properties to be referenced as defaults when a
component's SSL configuration does not provide either the key store or
trust store configuration.

For consistency with logback-core.

For consistency with SocketAppender.

Convenience constructors that are used to set mutable fields that have
proper accessors add little value, and they make it more complicated to
subclass.  Best practice is to use constructor arguments only to
initialize fields that are immutable or for which an accessor method
cannot be safely provided within the design constraints of the class.

The only use of these constructors by logback modules is in the
SocketMin class.  However, because these constructors were exposed as
API (public modifier) there is a chance that other code is using them,
therefore marking them as deprecated rather than removing them.

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