/v3/sortitions bad optimization for last_sortition_ch

[kantai]

last_sortition_ch is wrong in the /v3/sortitions endpoint. My recent consolidation PR changed this optimization:

// optimization heuristic: short-circuit the load if its just `stacks_parent_sn`
let last_sortition_ch = if sortition_sn.num_sortitions == stacks_parent_sn.num_sortitions + 1 {
    stacks_parent_sn.consensus_hash.clone()

To this:

// optimization heuristic: short-circuit the load if its just `stacks_parent_sn`
let last_sortition_ch = if stacks_parent_sn.sortition {
    stacks_parent_sn.consensus_hash.clone()

This is just wrong, and it leads to signers rejecting some proposals that they shouldn't (and accepting some proposals that they shouldn't).

[blockstack-devops]

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