[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.
Ok, so as I see it there are 3 options:

1) the return value is a hash of {:eid1 {:created-at ...} :eid2 {:created-at...}}. This will be guaranteed to only have at most one entry in the 2 parameter version, but not the 3 parameter. Makes dealing with the result a bit more complicated.

2) if more than 1 entity is found in the results, we just return an empty timestamp hash, as the "indeterminate" result. Could even add in that the attribute field has to be a UUID, this being the "public identifier" version of timestamps.

3) remove it and move on.

At this point, I'll accept 3 if you don't see 1 or 2 working, though I still think it could be useful functionality.