[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.
Yeah, I could put in stronger wording in the docstring. I did think about removing it, but it seems a shame to not be able to use it. For example, if we were to implement some web-caching mechanisms, and you wanted to know if some resource identified by a UUID has been updated, you could get the timestamps in one access instead of two.