Function search_in 

async fn search_in(
    session: &mut Imap,
    account: &Account,
    search: &Search,
    folders: &[Folder],
    here: &BTreeMap<String, BTreeSet<(u32, u32)>>,
    limit: usize,
    senders: Option<&Senders>,
) -> Result<Outcome, SyncError>