Queue of objects waiting to be uploaded to the store.
We don’t upload nodes directly, because it frequently happens that a node gets modified or deleted soon after it is created. It makes sense to wait a bit so we can avoid the costly upload operation.
This class holds the nodes in a queue, and uploads them if they get pushed out of the queue.
really_put is the function to call to really upload a node. max_length is the maximum number of nodes to keep in the queue.