[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.
Ah, but all* queries are local. So there's no roundtrip issues like with SQL.

Even with stronger wording in the docstring, the behavior of this fn would be "get me all the timestamps for all transactions involving *all the entities* that have *ever* in their history had this value for this attribute and then present them in this fashion which is misleading since we're dealing with n entities, not just one".

And, as I understand our present usecase, we already have the entity id by the time we'd want to answer this question.

\*mostly