async fn fetch_history(
api: &dyn SlackApi,
workspace: &str,
channel: &str,
oldest: Option<&str>,
) -> Result<Vec<SlackHistoryMessage>, SlackError>Expand description
Every history message after each page boundary, following the cursor.
One page is SWEEP_LIMIT messages, and a sweep that read only the first
would advance the watermark past everything it never fetched, losing
those messages silently.