Provide summary report of what is stored

[linas]

The (cog-report-counts) function provides a summary of the Atom types stored in the AtomSpace. A similar function is needed, to report what is held in a RocksStorageNode (without actually loading those atoms, of course).

The simple version of this would be to modify the existing report function to total these up and print them. The fancy version of this would be to also add the API to the generic StorageNode API, to make it usable by all backends.