constructor(graphData, { nodes, dependencies }) {
    super(graphData);

    // Nodes to insert.
    this.nodes = nodes;
    this.dependencies = dependencies;
  }
Inserts a batch of nodes for a GraphInsert.

One of these is created for each batch of nodes that can be inserted at once.
However, the nodes can have a different table and not all databases support
batch inserts, so this class splits the inserts into further sub batches
when needed.