A method of a linked class annotated with @LinkedTimer(delay, period) is run
repeatedly by the linker, on Spigot and Velocity. The method must not have
parameters, its return value is ignored. It is called through a MethodHandle
bound once when linking instead of reflection; LambdaMetafactory cannot be used
as the linker and the linked classes are in different class loaders.
The unit defaults to ticks and can be set to milliseconds up to hours; the
Unit enum converts it to ticks for Spigot and to a TimeUnit for Velocity.
HullHiderCommand uses it for its tick task.
The value of a @Linked field of a linked class is linked like a linked class
itself, e.g. registered as listener, and unlinked again on unlink. The field
has to be initialised when linking. HullHiderCommand and TPSSystem use it
instead of registering their listeners by hand.