Update index & offset encoding

Previously, the index & offsets were encoded as 8-character hex
strings, however, this limits the maximum value to a `uint32`. This
is normally not an issue, however, indices could go over the maximum
value of 4 billion over time and the offset could exceed this value
for an especially large WAL update. For safety, these encodings have
been updated to 16-character hex encodings.

db.go

@@ -703,7 +703,7 @@ func (db *DB) cleanWAL(ctx context.Context) error {
 			return err
 		}
 
-		db.Logger.Printf("remove shadow index: %s/%08x", generation, index)
+		db.Logger.Printf("remove shadow index: %s/%s", generation, FormatIndex(index))
 	}
 
 	return nil