This commit fully refactors the core campaign manager logic.
It applies a whole new approach to campaign state and lifecycle management.
- Create a new "pipeline" abstraction on top of campaign
for state management.
- Account for every message processed and end campaigns
based on the actual count.
- Discard in-queue messages in the pipeline of a paused
or cancelled campaign.
- `GET /api/events?type=error` opens a long-lived HTTP server side
event connection that streams error messages.
- async (typically SMTP) errors are now streamed to the frontend and
disaplyed as an error toast on the admin UI.