- sync_grow now uses the date probing mechanism we added for vanilla IMAP so
  that we will try and sync the entire folder if viable and otherwise take
  a smart bite of the messages in the folder.
- we track localMessageCount to support that use-case.  It may also be
  interesting and useful on its own.
- gmail now reports fullySynced just like vanilla IMAP
- we mark the all mail/junk/trash folders for gmail as !selectable to paper
  over various implementation limitations.