Update gateway to use DB transactions for Plan Import

[dandelany]

Background

Low-priority follow-up to NASA-AMMOS/aerie-gateway#109 - this PR includes a “faux transaction” ie. our code rolls back DB state if anything goes wrong during import to avoid corruption. This is theoretically almost as safe as a DB transaction, unless gateway dies/errors halfway through.

Description

We'd like to refactor the gateway to use "real" DB transactions for Plan Import (& potentially other similar operations). However this is complicated by two things:

• Hasura's GraphQL API/engine does does not have proper transaction support - though there is an issue + RFC proposal
  • this can be worked around by having Gateway connect directly to the Postgres service and using DB transactions directly, except that...
• We need to revisit DB permissions and roles to give Gateway the proper permissions, so Revisit Postgres DB user permissions, roles and groups #1576 is a blocker for this