Use secondary storage for network graph

[tnull]

Now that we're moving towards supporting remote storage in general and VSS in particular, we should consider discerning which data could/should still live only in a local data store. One such example would be NetworkGraph, that is if the network graph remains only an object storing gossip data, i.e., would only ever contain data that we can easily resync from the network.

[G8XSU]

Imo, in addition to network_graph, we should also explore more granular control over using different types of kv_stores.

For example not all stores need remote_first storage, some might be fine with local_fist, some could be better suited for sql backend. I might want to store my network_graph and scorer in object store like s3 etc.