Add lighthouse db command (#3129)

## Proposed Changes

Add a `lighthouse db` command with three initial subcommands:

- `lighthouse db version`: print the database schema version.
- `lighthouse db migrate --to N`: manually upgrade (or downgrade!) the database to a different version.
- `lighthouse db inspect --column C`: log the key and size in bytes of every value in a given `DBColumn`.

This PR lays the groundwork for other changes, namely:

- Mark's fast-deposit sync (https://github.com/sigp/lighthouse/pull/2915), for which I think we should implement a database downgrade (from v9 to v8).
- My `tree-states` work, which already implements a downgrade (v10 to v8).
- Standalone purge commands like `lighthouse db purge-dht` per https://github.com/sigp/lighthouse/issues/2824.

## Additional Info

I updated the `strum` crate to 0.24.0, which necessitated some changes in the network code to remove calls to deprecated methods.

Thanks to @winksaville for the motivation, and implementation work that I used as a source of inspiration (https://github.com/sigp/lighthouse/pull/2685).

beacon_node/store/src/hot_cold_store.rs

@@ -191,9 +191,8 @@ impl<E: EthSpec> HotColdDB<E, LevelDB<E>, LevelDB<E>> {
             info!(
                 db.log,
                 "Hot-Cold DB initialized";
-                "version" => CURRENT_SCHEMA_VERSION.as_u64(),
                 "split_slot" => split.slot,
-                "split_state" => format!("{:?}", split.state_root)
+                "split_state" => ?split.state_root
             );
         }