Function drain 

pub async fn drain(
    client: &Client,
    store: &ProtectedLocalKeyStore,
    database: &Database,
    host: &dyn ClientHost,
    blob_dir: &Path,
    round_limit: usize,
) -> Result<Outcome>

Repeats bounded rounds until metadata is current and image work settles, or until round_limit or the image retry bound stops it.