add a db_ts_get_latest_validity

[HomoCodens]

db_ts_get_last_update has its uses but recently it has come to light that some use cases actually require the latest stored validity i.e. vintage date, not simply the timestamp of when the series was stored to the database.

We should add a db_ts_get_latest_validity and possibly db_ts_get_validities.

n.b. that metadata even already has such a function with db_meta_get_latest_validity