Custom listeners / authorizers

[uklance]

• Abstract the singleton Maps from CometdPushManager into a separate service (CometdGlobals)
• Support connect listeners (topic based)
• Support disconnect listeners (topic based)
• Support authorizer contributions (topic based)
• Perhaps refactor CometdPushManager into listeners, delete the class and move everything to CometdModule.buildBayeuxServer()

public interface SubscriptionListener {
   public void onSubscribe(String topic, ClientContext context);
   public void onUnsubscribe(String topic, ClientContext context);
   public String getTopic();
}

public interface SubscriptionListeners {
   public void addListener(SubscriptionListener listener);
   public void removeListener(SubscriptionListener listener);
   public List<SubscriptionListener> getListeners(String topic);
}

public interface Authorizer {
   public boolean authorize(String topic, ClientContext context);
   public String getTopic();
}

public interface Authorizers {
   public void addAuthorizer(Authorizer auth);
   public void removeAuthorizer(Authorizer auth);
   public List<Authorizer> getAuthorizers(String topic);
}

[uklance]

Related to #24

[uklance]

Authorizers implemented (not tested)