Jepsen Postgres tests

This repo includes tests for several related systems that expose a Postgres API. The core tests for single-node Postgres are in postgres/. Tests for AWS RDS Postgres clusters are in rds/.

We package these separately because some Postgres-based tests pull in conflicting dependencies. Stolon, for instance, uses jetcd, which uses a bunch of Jetty deps. RDS needs the AWS API, which also uses Jetty, but incompatible versions.

This also makes it possible to pull in the Postgres tests as a library when testing other Postgres-based systems, like Cockroach.