Fix `RocksDBStore.IterateIndexes()`

The internal logic of `IterateIndexes()` is bugged, which is why #3999 uses a different implementation to bypass the bugged behavior of `IterateIndexes()`. In particular, `chainInfo.Item2` should be the number of `Block`s associated with a chain id, **not** the `Tip.Index` (which differs by 1). I am not sure as to how this hasn't been causing more serious problems (at least seemingly) so far, but upon inspection, it's been confirmed that the internal database already has been corrupted. After #3999, it should become rather trivial to fix this issue.