public List<Node> write(List<Node> nodes, @SuppressWarnings("unused") Mutex lock) {
        return db.writeTo(nodes, Agent.system, Optional.empty());
    }
Writes these nodes after they have changed some internal state but NOT changed their state field.
This does NOT lock the node repository implicitly, but callers are expected to already hold the lock.

@param lock already acquired lock
@return the written nodes for convenience
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/node/Nodes.java