Fix issues with fetching emails from vector expansion
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.
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