lib-storage: convert dbox-common to use container_of

dbox: Use mail_index_header.last_temp_file_scan instead of directory's atime This will be required for the following ITERINDEX change.

sdbox: Don't create index directory when trying to open nonexistent mailboxes

mdbox: Fixed race condition when creating a new mailbox and another process getting its GUID.

dbox: Don't cache pop3.uidl|order unless index header indicates there are those. They exist only when doing a migration, so it's pretty wasteful storing "doesn't exist" for all other installations.

Replaced *_INDEX_PREFIX macros with a common MAIL_INDEX_PREFIX.

dbox: Renamed dbox_sync_rebuild_verify_alt_storage() to dbox_verify_alt_storage()

Added mail_temp_scan_interval setting and changed its default from 8h -> 1w.

dbox: Create symlink to alt root dir, and warn whenever it changes.

lib-storage: Added support for saving mail attachments separately via filesystem API. Currently this works only with sdbox and mdbox backends.

dbox: Create dbox temp files with ".temp." prefix, not "temp." This makes it consistent with the rest of the created temp files and also gets any stale temp files deleted.

[m]dbox: If alt-dir exists for a mailbox while trying to create it, rebuild mailbox indexes. This makes sure that if there are any mails in the alt-dir, they become visible rather than be overwritten. --HG-- branch : HEAD

dbox, mdbox: Fixed race conditions when creating mailboxes. --HG-- branch : HEAD

mdbox: Avoid rebuilding storage if another process already did it. --HG-- branch : HEAD

mdbox: Automatically delete old temp.* files from storage/ directory. --HG-- branch : HEAD

dbox: Moved SDBOX_INDEX_FLAG_ALT macro to dbox-common. --HG-- branch : HEAD

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

lib-storage: Mailbox renaming API changed. --HG-- branch : HEAD

lib-storage: Mailbox deletion API changed. Mailbox deletion should now be free of race conditions. The actual file deletion code is now responsibility of mailbox_list backend. --HG-- branch : HEAD

lib-storage: Removed mailbox_list_get_guid(). Directory GUIDs are pretty pointless. --HG-- branch : HEAD

Split dbox (single-dbox) and mdbox (multi-dbox) into separate storage backends. This cleans up the code, makes it faster and also fixes some bugs. Super-fast maildir migration code was also dropped, at least for now. --HG-- branch : HEAD