Renamed events to messages in the PostgreSQL and SQLite schemas and added message kind column

[oskardudycz]

A follow-up to adding a general message structure in #183.

As we want to store not only events but in general messages, it'd be good to align table naming structure. This PR renames tables and columns to have event in their names instead of message (e.g., message ID, message type, etc.).

Thanks to that, Emmett will be able to store and then handle async workflows as described in https://event-driven.io/en/how_to_have_fun_with_typescript_and_workflow/. Commands will be stored either in the stream together with events or in a dedicated "outgoing commands stream".

This PR changes PostgreSQL and SQLite schema in a breaking manner:

• emt_events is migrated into emt_messages (and related partitions tables),
• emt_append_event function in PostgreSQL is renamed to emt_append_to_stream,
• emt_global_event_position PostgreSQL sequence into emt_global_message_position

But:

• For PostgreSQL automated migration was added,
• For SQLite, there's no one, afaik, using it on prod yet, so no migration here.

[oskardudycz]

@davecosec FYI.

[oskardudycz]

Okay, I'm pulling that in. I checked if the PostgreSQL migration would work on the sample project, and it did.