refactor: create EmailInformationCache

[gbrodman]

not quite sure how this will work, but order info, cancellations, and eventually refunds will be stored in a format where we have a list of email IDs, then a map<email_id, some_information> where we retrieve the whole thing from Drive, and only load from the emails themselves if the email_id doesn't exist in the Drive map. We should refactor this so the code is unified.