maildir: Added maildir_empty_new setting to avoid keeping mails in new/ directory. This is useful for example when using an external MDA that calls "doveadm index" and the user hasn't opened the mailbox for a long time. Also this removes the delay that user may see when opening a rarely accessed mailbox, which renames a ton of files to cur/ while the user is waiting.

maildir: Added maildir_broken_filename_sizes setting.

maildir: Always try to preserve message GUIDs when copying. Because of this, maildir_copy_preserve_filename setting is now removed. --HG-- branch : HEAD

lib-storage: Changed mailbox_list.iter_is_mailbox() API. --HG-- branch : HEAD

Initial commit for config rewrite. --HG-- branch : HEAD