Add commmand for pruning states (#4835)

## Issue Addressed

Closes #4481. 

(Continuation of #4648)

## Proposed Changes

- [x] Add `lighthouse db prune-states`
- [x] Make it work
- [x] Ensure block roots are handled correctly (to be addressed in 4735)
- [x] Check perf on mainnet/Goerli/Gnosis (takes a few seconds max)
- [x] Run block root healing logic (#4875 ) at the beginning
- [x] Add some tests
- [x] Update docs
- [x] Add `--freezer` flag and other improvements to `lighthouse db inspect`

Co-authored-by: Michael Sproul <michael@sigmaprime.io>
Co-authored-by: Jimmy Chen <jimmy@sigmaprime.io>
Co-authored-by: Michael Sproul <micsproul@gmail.com>

beacon_node/store/src/leveldb_store.rs

@@ -169,6 +169,7 @@ impl<E: EthSpec> KeyValueStore<E> for LevelDB<E> {
         for (start_key, end_key) in [
             endpoints(DBColumn::BeaconStateTemporary),
             endpoints(DBColumn::BeaconState),
+            endpoints(DBColumn::BeaconStateSummary),
         ] {
             self.db.compact(&start_key, &end_key);
         }
@@ -225,9 +226,9 @@ impl<E: EthSpec> KeyValueStore<E> for LevelDB<E> {
     }
 
     /// Iterate through all keys and values in a particular column.
-    fn iter_column_keys(&self, column: DBColumn) -> ColumnKeyIter {
+    fn iter_column_keys<K: Key>(&self, column: DBColumn) -> ColumnKeyIter<K> {
         let start_key =
-            BytesKey::from_vec(get_key_for_col(column.into(), Hash256::zero().as_bytes()));
+            BytesKey::from_vec(get_key_for_col(column.into(), &vec![0; column.key_size()]));
 
         let iter = self.db.keys_iter(self.read_options());
         iter.seek(&start_key);
@@ -235,13 +236,12 @@ impl<E: EthSpec> KeyValueStore<E> for LevelDB<E> {
         Box::new(
             iter.take_while(move |key| key.matches_column(column))
                 .map(move |bytes_key| {
-                    let key =
-                        bytes_key
-                            .remove_column(column)
-                            .ok_or(HotColdDBError::IterationError {
-                                unexpected_key: bytes_key,
-                            })?;
-                    Ok(key)
+                    let key = bytes_key.remove_column_variable(column).ok_or_else(|| {
+                        HotColdDBError::IterationError {
+                            unexpected_key: bytes_key.clone(),
+                        }
+                    })?;
+                    K::from_bytes(key)
                 }),
         )
     }