eventually-postgres: Overhaul after v0.4.0 breaking changes

[ar3s3ru]

The current state of eventually-postgres is pretty bad, being disabled after #81 has been merged.

Things that are currently missing in the crate:

• Proper schema for aggregates/events (eventually-postgres: library overhaul after latest changes #87)
• Schema migration tool to run at startup (use https://github.com/rust-db/refinery) (eventually-postgres: library overhaul after latest changes #87)
• Move sequence numbering and versioning to the database (using triggers or functions?) (eventually-postgres: library overhaul after latest changes #87)
• Use NOTIFY/LISTEN for creating persistent streams

Schema for Aggregates and Events

The database should potentially support a multi-Aggregate setup, so the proposed tables layout looks like this:

Aggregate Types

Contains all the Aggregate types definition in a centralized place.

Column Name	Description
id	Type identifier of the Aggregate -- might use type_name
offset	Global offset of the $all stream.

Aggregates

Contains all the Aggregates created by the application.

Column Name	Description
id	Aggregate ID -- should be unique and a string.
aggregate_type_id	Reference to the aggregate_types table. Useful for accessing the Aggregate Type offset.
version	Last version of the Aggregate.

Events

Column Name	Description
aggregate_id	Reference to the aggregates table.
version	Version of the Event, for Optimistic Concurrency.
sequence_number	Value of the Global Offset at the time of insert -- useful for $all streams publishing idempotency.
event	Contents of the event -- should be a JSON.

Notes

• (aggregate_id, version) should be unique.

[ar3s3ru]

Use NOTIFY/LISTEN for creating persistent streams

This will be done as part of the Subscription implementation. More info to come.