Provide query "is this Atom being held in this DB?"

[linas]

The (cog-node TYPE NAME) function allows a user to ask "does a node with the given TYPE and NAME exist in the AtomSpace?", without actually creating that Node. A similar function is needed for the RocksStorageNode: to ask if an Atom is being held, without actually creating it.

This needs modifications the generic StorageNode API, so that this question can be asked w/ the generic API, which then trickles down to the specific StorageNode.

In the same vein, other useful functions would be modeled on (cog-incoming-size ATOM) and (cog-incoming-size-by-type ATOM TYPE) to report on the incoming set of an ATOM held in storage, without actually fetching that incoming set.