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,
}