PostgreSQL LISTEN/NOTIFY Subscriber

The same as #5, but for PostgreSQL :)

This implementation should be added in https://github.com/ThreeDotsLabs/watermill-sql/ repository. It need to be compatible with already existing SQL Subscriber.

That means, that you should use SQL publisher for publishing messages, and binlog subscriber to read them. We would, of course, keep already existing SQL based subscriber.

I'm not expert in PostgreSQL area, so any comments and ideas are welcome! :wink:

It probably depends on #127.