[Chunk Data Pack Pruner] Add Block Iterator

[zhangchiqing]

This PR adds a height based block iterator that iterates blocks by height, without iterating siblings of finalized blocks, which will be done later by implementing view based block iterator.

[zhangchiqing]

The BlockIterator interface can be implemented into height based iterator and view based iterator.

The block iterator not long can be used by chunk data pack pruner, but alsoin future to implement protocol state pruner.

The height based iterator is easy to implement, however, it can't guarantee to prune all data, since it doesn't iterate unfinalized blocks. The view based iterator can guarantee all blocks are pruned, but it's more complicated to implement.

In this PR, I first implement the height based iterator, for chunk data pack, it's OK that we only prune by height, however, for protocol state, it's better that we can prune by view and ensure a more throughout pruning.

[zhangchiqing]

Once the interfaces in the arguments are implemented, then the logic to create the BlockIterator can be reused. That's why, I put this function here along with the interface definitions, so that it's clear to see how the interfaces will be used for creating the block iterator.