FR: Allow GetBlockRange to stream blocks in descending order

[adityapk00]

What is your feature request?
Right now, the GetBlockRange method streams blocks from lowest height to tallest height. For BlazeSync, it might be useful to have the blocks streamed backwards - i.e., tallest height to lowest height. Maybe have an optional parameter that specifies the direction in which to stream blocks

How would this feature help you?
Streaming blocks backwards allows wallets to detect incoming spends quicker, allowing users access to their most recent transactions first.

cc: @LarryRuane , @braddmiller

[LarryRuane]

This should be easy, just generalize GetBlockRange() to run the loop index either up or down. Rather than adding an optional parameter, it may make sense to just allow start=20, end=10 to mean return 20, 19, ... 10. Currently, those arguments would return an empty list. This approach is almost backward-compatible -- close enough, I think.

[adityapk00]

This is a great idea. We should just do this.

[braddmiller]

This looks like a really great solution, adding to our sprint

[braddmiller]

@mdr0id, can we get PR #355 up in our LWD instance for testing?