Better event store abstractions

[lazamar]

This PR creates

• A minimal interface for an EventStore
• An abstraction to the connection to a Postgres database
• An abstraction to a Postgres transaction
• An abstraction to a Postgres event store, which is built on top of an open Postgres transaction.

This creates a much clearer separation of concerns between these abstraction layers.
These abstractions are not being used yet because I need to refactor the handlers first.