add commit cache (feature #622)

yes. we have `getNodes` / `getNodesFromDb` and `setNodes` / `setNodesToDb`. If you change the last one to saveNodesToDb, then you should also change setNodes to saveNodes, and then you should also change getNodes to loadNodes, etcetera.

All those words "set", "save", "commit" mean something in English, but have acquired a special meaning in programming.

In natural English you would probably say "I set the nodes in the database" instead of "I set the nodes to the database", so I changed it to `setNodesInDb`