Follows #528.

It would be great to be able to run multiple nodes in parallel.

While starting two nodes is easy (basically just create multiple asyncio tasks), and defining multiple nodes to run is easily (return a list of nodes instead of a single node), some other stuff is hard:

• how do we manage recording state (see State persistence #695) when multiple nodes run in parallel?
• how to we know when to start a node that relies on multiple other nodes completing?
• currently nodes define just their outgoing edges, do we need nodes to define their incoming edges too?