Add support for pausing an actor

[ctrlpz]

• add static map in in Tracing actors to keep all the actors created in the system. One actor is added to the map every time an actor is created by the createActorFromValue primitive.
• add a new incomming message for the request of pausing an actor (PauseActorRequest)
• add a method in the WebDebugger that get the actor by its id accessing the map created in TracingActor class
• make public the log method of the FrontendConnector to notify by console the actor will pause before processing the next message