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.
The generics pose a problem, and the parameter passed to the
Consumer is not needed in API.
Additionally, stop trying to cancel Bukkit scheduler tasks on
plugin disable as the Bukkit scheduler does not work.
