WIP in https://github.com/taskcluster/taskcluster/pull/79

I talked to John about his plans, and he's got some work on a postgres-entities library already in progress that sounds pretty neat. So I'll set this stored-procedure approach aside and take a look at postgres-entities.

I still need to get some terraform together to set up a DB and provide URLs for it.

Notes on using Postgres in GKE:

• Cloud SQL supports Postgres 9.6 (so we shouldn't be targeting anything newer than that)
• It supports both "public IP" and "private IP" modes. Obviously, we'd prefer the latter. That seems to require the "Service Networking API" be enabled, and an IP range allocation (which the console helpfully does for you). Docs suggest that private IP mode works from GKE. This seems to connect to the default VPC.
• GKE must be "VPC-native", which our clusters aren't (but this will soon become default, says the UI, so we might as well..)

With all of that in place, I was able to start a k8s Job that could connect to a test database. So, all of the pieces are in place.

I'm going to call this finished for now. When it's more clear that postgres is coming, I can work on the terraform required to deploy this. In the interim, I'll work on getting clusters redeployed to be vpc-native.

^^ bug 1522675