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/leveldb_store.rs

@@ -1,4 +1,5 @@
 use super::*;
+use crate::hot_cold_store::HotColdDBError;
 use crate::metrics;
 use db_key::Key;
 use leveldb::compaction::Compaction;
@@ -6,7 +7,7 @@ use leveldb::database::batch::{Batch, Writebatch};
 use leveldb::database::kv::KV;
 use leveldb::database::Database;
 use leveldb::error::Error as LevelDBError;
-use leveldb::iterator::{Iterable, KeyIterator};
+use leveldb::iterator::{Iterable, KeyIterator, LevelDBIterator};
 use leveldb::options::{Options, ReadOptions, WriteOptions};
 use parking_lot::{Mutex, MutexGuard};
 use std::marker::PhantomData;
@@ -174,6 +175,28 @@ impl<E: EthSpec> KeyValueStore<E> for LevelDB<E> {
         }
         Ok(())
     }
+
+    /// Iterate through all keys and values in a particular column.
+    fn iter_column(&self, column: DBColumn) -> ColumnIter {
+        let start_key =
+            BytesKey::from_vec(get_key_for_col(column.into(), Hash256::zero().as_bytes()));
+
+        let iter = self.db.iter(self.read_options());
+        iter.seek(&start_key);
+
+        Box::new(
+            iter.take_while(move |(key, _)| key.matches_column(column))
+                .map(move |(bytes_key, value)| {
+                    let key =
+                        bytes_key
+                            .remove_column(column)
+                            .ok_or(HotColdDBError::IterationError {
+                                unexpected_key: bytes_key,
+                            })?;
+                    Ok((key, value))
+                }),
+        )
+    }
 }
 
 impl<E: EthSpec> ItemStore<E> for LevelDB<E> {}