+ - notify_sync() could have "what changed" struct with old/new flags
- mailbox deletion could really be "rmdir", which fails if everything isn't
already expunged. IMAP DELETE could fail with "mailbox changed during
deletion, aborting" if new messages arrive
+ - if doing any changes, figure out how to handle symlinked mailboxes
- maildir: copy dovecot-shared file from parent mailbox, not root.
- is mail_index_sync_have_any() really necessary? especially that it returns
TRUE for ext rec changes ... why?...