Registers a block of code (callback) to run after ActorSystem.shutdown has been issued and all actors in this actor system have been stopped. Multiple code blocks may be registered by calling this method multiple times.

The callbacks will be run sequentially in reverse order of registration, i.e. last registration is run first.

Terminates this actor system. This will stop the guardian actor, which in turn will recursively stop all its child actors, then the system guardian (below which the logging actors reside) and the execute all registered termination handlers (RegisterOnTermination(Action)).

Be careful to not schedule any operations on completion of the returned task using the `dispatcher` of this actor system as it will have been shut down before the task completes.