[Node Manager] Treats min_nodes as min_nodes_idle when job queue is empty
Two Node Manager issues were resolved around the same time: spin up new nodes when current nodes are all busy (#4357), and supporting a min_nodes configuration knob (#4294). Unfortunately, the implementations don't play well together. Because min_nodes is implemented by making sure the server wishlist has at least min_nodes entries, when all up nodes are busy, the daemon believes that it must start a new one to satisfy the wishlist, and does so.
#4299 will require a richer representation of the wishlist anyway, so it might be reasonable to go ahead and do that now to fix this bug.