lib-storage: Try to add missing attachment flags when fetching MIME parts or BODYSTRU...

lib-storage: Try to add missing attachment flags when fetching MIME parts or BODYSTRUCTURE

This happens only if both mime.parts and imap.bodystructure are already in
cache and mail_attachment_detection_options has add-flags-on-save but no
no-flags-on-fetch. The no-flags-on-fetch option may be removed in a future
release once it's known that it's not causing any unexpected performance
issues.