[TH] timestamps function

Assigning to @tie-rack 

Utility function for getting timestamps for an entity. You pass in an (ideally) attribute and a value that can determine the entity, and get back a hash with :created-at, :updated-at, and :timestamps which are all the modification times in ascending order.

I'd like to make a version that works with :db/id, but there's something special about :db/id that I haven't sorted out how to specify it properly yet, and this will work in the meantime for most of our entities.

No particular card for this, it'll get used in some ballot-scout work.
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```clojure
(defn timestamps
  "Returns a hash with keys :created-at, :updated-at, and :timestamps,
   the latter being all the timestamps of transactions affecting the
   entity in ascending order.  Pass in the database and entity id."
  [db e]
  (let [timestamps (->> (d/q '[:find ?txInstant
                               :in $ ?e
                               :where
                               [?e _ _ ?t]
                               [?t :db/txInstant ?txInstant]]
                             (d/history db) e)
                        (map first)
                        sort)]
    {:created-at (first timestamps)
     :updated-at (last timestamps)
     :timestamps timestamps}))
```