imapx_store_get_folder_info_sync() cleanups.
CamelIMAPXStore: Remove unused public members.
CamelIMAPXStore: Move folder info refresh stuff to private section.
CamelIMAPXStore: Obtain a fresh folder list on startup.

Unless we're offline, always obtain a fresh mailbox listing from the
IMAP server on startup.  The result of get_folder_info_sync() should
always be as complete and accurate as possible.