TaskStatusDB: Rebuild task network object

[dwhswenson]

Go from the databases to an nx.DiGraph of Tasks.

This isn't strictly necessary for minimal functionality, but has potential to be very useful for things like troubleshooting, debugging, and ensuring database consistency.

This should actually be done in 2 stages: going from the TaskStatusDB to a network of taskid strings, and then a second function that takes that taskid network and attaches TaskDetails from the TaskDetailsStore.