Add "last updated" item to DB

[jsstevenson]

In PG, something like a "meta" table. Can just be a simple object in Dynamo.

Conceivably could hold other info as well.

[korikuzma]

@jsstevenson To confirm, would this contain the timestamp when we last updated the db?

[jsstevenson]

Thinking about this more --

• it'd be nice to have a way to mark that a DB currently contains a published snapshot. E.g., if I load a data dump from S3, a line in all responses could indicate that I'm working out of that specific data instance. Then, if I alter or update the data tables at all, that line is rendered null. This would/could be distinct from a "last updated" value.