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/src/lib.rs

@@ -13,7 +13,7 @@ use beacon_chain::{
 use clap::ArgMatches;
 pub use cli::cli_app;
 pub use client::{Client, ClientBuilder, ClientConfig, ClientGenesis};
-pub use config::{get_config, get_data_dir, set_network_config};
+pub use config::{get_config, get_data_dir, get_slots_per_restore_point, set_network_config};
 use environment::RuntimeContext;
 pub use eth2_config::Eth2Config;
 use slasher::Slasher;