Extract "scan arbitrary block range" function from scan_cached_blocks

[str4d]

scan_cached_blocks has a specific interaction pattern with other chain scanning functions like validate_chain, and is built around an assumption of linear scanning. For DAGSync we want to be able to perform "random-access scanning", with the ability to handle contiguous batches of blocks (to optimise network access patterns and note commitment tree update patterns).

As a stepping stone towards this, we want to enable scanning the last 100 blocks as a batch (this being the section of the chain that can encounter reorgs during steady state operations). Rather than expanding or adapting scan_cached_blocks for this purpose, we should extract a new function that can perform this operation.

This may or may not be reusable for DAGSync itself (where we will be using a BatchScanner that processes more than just a contiguous batch), but we can adapt it at that point (vs scan_cached_blocks which should have its semantics preserved until we can remove it entirely).

[nuttycom]

This was completed by the changes in #831 and #872