Migrate background queues to Knative Eventing

[gnarea]

Describe the problem

NATS Streaming is problematic in production (relaycorp/cloud-gateway#84) so our background queues should use GCP PubSub in production, whilst continuing to use NATS Streaming in development.

Describe the solution you'd like

A broker-agnostic system to manage background queues, so that we can use different systems in production and development -- like Knative Eventing, which could also up/down scale workers according to load.

Additional context

This isn't the only use of NATS Streaming. See #929

[gnarea]

Knative Eventing is a horrible pain in the neck and we don't actually need it thanks to @relaycorp/cloudevents-transport