Use Postgresql for the Merchant database

[marsella]

sqlx has difficulty validating queries against multiple database types when you run cargo sqlx:prepare, which we'd need to address. See launchbadge/sqlx#121

[yaymukund]

@marsella What does this entail?

[marsella]

I don't know, just saw this yesterday. I think that I might have created it when we did the triage. If you don't remember what it was for either, we can probably close it.

[yaymukund]

I think this was meant as a dumping ground for longer-term db improvements. I've made it more specifically about using postgresql for the merchant db, since #99 is already ticketed.