Changes

Process entity portalling towards the end of a tick.
Cross world teleportation works by taking a copy of an entity and moving
it to a new world. After this happens the original entity is marked as
dead so as to be removed from the original world, however it still
undergoes one further tick in the main world, but with some information
from the new world. It is not so easy to break out of this tick cycle if
needed, so instead we move the portalling process towards the end of an
existing tick. This ensures that the entity will not be spuriously
ticked.