Remove use of `Option<BlockHeight>` as a query for blocks

[MitchTurner]

There are a number of places we use Option<BlockHeight> to represent the block, where None is the genesis block. This is confusing, as it's implicit what None means without looking at the code.

Instead, we should introduce some type with the same cardinality, but with explicit names:

something like

enum BlockQuery {
    Specific(BlockHeight),
    Genesis,
}

[abdegenius]

Hey @MitchTurner i can work on this issue.. should i go ahead and take it up?!

[MitchTurner]

@abdegenius Hey! I believe that @matt-user has already dug into this here. It looks like it's still blocked on some reviews.

[abdegenius]

Oh ok then, let me know if it is still a problem so i can jump on it.