Fix: /v3/sortitions calculation and tenure extend behavior

[kantai]

This fixes two issues that came up in nakamoto testnet (probably only #5252 came up so far -- still investigating the details of #5248)

[wileyj]

maybe the target branch here should be the rc branch? then we would PR the rc branch back to develop.
I do have some unmerged docs around this in a different PR. i'll see if i can push that forward.

[kantai]

The four failing tests in CI:

tests::signer::v0::min_gap_between_blocks — passes locally
tests::signer::v0::partial_tenure_fork — passes locally
tests::signer::v0::bitcoind_forking_test — passes locally
tests::signer::v0::multiple_miners_with_nakamoto_blocks — fails in develop

[obycode]

LGTM

[blockstack-devops]

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