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/lighthouse_network/src/rpc/methods.rs

@@ -9,7 +9,7 @@ use ssz_types::{
     VariableList,
 };
 use std::ops::Deref;
-use strum::AsStaticStr;
+use strum::IntoStaticStr;
 use superstruct::superstruct;
 use types::{Epoch, EthSpec, Hash256, SignedBeaconBlock, Slot};
 
@@ -263,7 +263,7 @@ pub enum RPCCodedResponse<T: EthSpec> {
 }
 
 /// The code assigned to an erroneous `RPCResponse`.
-#[derive(Debug, Clone, Copy, PartialEq, AsStaticStr)]
+#[derive(Debug, Clone, Copy, PartialEq, IntoStaticStr)]
 #[strum(serialize_all = "snake_case")]
 pub enum RPCResponseErrorCode {
     RateLimited,