Abstract base class for code that seeks to avoid pinning of webapp classloaders by using the jetty classloader to
proactively call the code that pins them (generally pinned as static data members, or as static
data members that are daemon threads (which use the context classloader)).
Instances of subclasses of this class should be set with Server.addBean(), which will
ensure that they are called when the Server instance starts up, which will have the jetty
classloader in scope.