Vector expansion was broken in that the unfulfilled map was being
populated with EmailIdentifiers lacking a message_id, only a UID
(from Imap.Folder).  More rigorous unfulfilled map now in
AbstractListEmail that requires a UID, not an EmailIdentifier.
Also guarantees that the email is fetched exactly once per
transaction by mapping UID -> fields, then "flipping" the map
back to fields -> UIDs, maximizing the number of emails requested
per FETCH.