Now, entity/global/location schedulers implement a generic run,
runDelayed, and runAtFixedRate methods that provide a ScheduledTask
value that can be used to interact with the scheduled task.
Add also an async task scheduler that implements the same methods,
except the delays/periods are in time and not ticks, as the scheduler
is independent of the server tick process.
Additionally, throw on some unimplemented APIs now.
Mid-tick task execution acquires the ticket lock at least,
which can possibly be a significant performance bottleneck
at a high tick thread + region count. This change should reduce
the impact from scaling the region threads, but is not a fix
to the underlying issue.
