db_ts_find_keys returns a data.table

[HomoCodens]

I'd have expected a vector.

[HomoCodens]

this is probably already in use somewhere so such a change would fall under "breaking changes"

on the other hand a 1 column data.frame is basically a vector and it saves having to access the keys in a separate step

[HomoCodens]

check for consistency with e.g. listing keys in dataset