Mark tasks as setting_up when their claiming node enters setting_up

[ranweiler]

When a node state enters setting_up, it does so because it has claimed a work set, which encompasses 1 or more tasks that share a common target setup container.

Right now, when this happens, the task state is scheduled, even though it has been claimed by a node. We don't reflect this until a given task is actually running, which may not happen until after running a target setup script, rebooting, &c. We also don't link the node and task commitment until this point, even though it exists earlier.

Instead, include the precipitating work set (or at least its task IDs) in the Node setting_up event, and immediately (1) update the task state to setting_up (which will be a new TaskState), and (2) create the NodeTask value which makes it known that the given task has been claimed by the given node.

[ranweiler]

Done in #43.