The ForkJoinScheduler is backed by a lightweight
fork-join task execution framework
This scheduler class uses a ThreadPoolExecutor
to execute Actors
The abstract SchedulerService class allows subclasses
to implement a custom onShutdown method, which is
invoked when the runtime system has detected that all actors have
been terminated
The SimpleExecutorScheduler class uses an
ExecutorService to execute Actors
This scheduler executes actor tasks on the current thread
The TerminationService class starts a new thread
that is used to check regularly if the scheduler can be
shut down, because all started actors are known to
have terminated
The ThreadPoolScheduler class uses a
ThreadPoolExecutor to execute Actors
Default scheduler for actors with daemon semantics, such as those backing futures