This is sort-of the combination of `/script/:type/:payload/confirmed-txs` and `/script/:type/:payload/unconfirmed-txs`, except sorted in reverse-chronological order.
We start pages at the most recent mempool tx, go backwards in the mempool until we reach the oldest tx in the mempool, then continue with the most recent DB tx and go backwards from there.
Note that unlike `confirmed_txs` and `unconfirmed_txs`, the order of txs observed by fetching multiple pages can change if new txs are added, or the page size is changed. This is because txs are fetched from the DB the order they appear on the blockchain, and only then are sorted by time_first_seen.