PerTaskState Reserved Key

[jspenger]

PerTaskState uses a reserved key for the moment for the Task State, see https://github.com/portals-project/portals/blob/main/core/src/main/scala/portals/tasks/TaskStates.scala#L50. This is strange, and will cause a bug if a key has the same value as the reserved key. We should ensure that the PerTaskState is always disjoint from the PerKeyState, through some more resilient means.