[Forknet] Copy out State and FlatState instead of deleting all other columns

[robin-near]

In the fork-network code:

    /// Deletes DB columns that are not needed in the new chain.
    fn finalize(&self, near_config: &mut NearConfig, home_dir: &Path) -> anyhow::Result<()> {
        // Open storage with migration
        let storage = open_storage(&home_dir, near_config).unwrap();
        let store = storage.get_hot_store();

        tracing::info!("Delete unneeded columns in the original DB");
        let mut update = store.store_update();
        for col in DBCol::iter() {
            match col {
                DBCol::DbVersion | DBCol::Misc | DBCol::State | DBCol::FlatState => {}
                _ => update.delete_all(col),
            }
        }
        update.commit()?;
        Ok(())
    }

Deleting the columns probably do not save a ton of space, or at least would need a lengthy compaction step. Instead, we can do the following:

• Copy DbVersion, Misc, and FlatState to a separate database
• Copy only the values in State (not the nodes) to this separate database
• Swap the new database in place.

With new research, this process should take about only 2 minutes to complete, and should leave a database that is quite tiny in size (30GB ish?)