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.
This may be useful for plugins which want to perform operations
over large areas outside of the buffer zone provided by the
regionaliser, as it is not guaranteed that anything outside
of the buffer zone is owned. Then, the plugins may use
the schedulers depending on the result of the ownership
check.
