lib-storage: convert cydir to use container_of

Replaced *_INDEX_PREFIX macros with a common MAIL_INDEX_PREFIX.

Avoid including mailbox-list-private.h everywhere. --HG-- branch : HEAD

lib-storage: *_mailboxes don't descend from index_mailbox anymore, it's now a context. --HG-- branch : HEAD

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

Transaction commits can now track how many uid/modseq updates were ignored. --HG-- branch : HEAD

Added mailbox_list_get_guid() to get/create directory GUID. Directory GUIDs are mainly useful for dsync to identify \Noselect mailboxes. --HG-- branch : HEAD

index-storage: Simplified the way transactions are are created/finished. Removed also mail_storage.class_init/deinit since they're no longer used for anything. --HG-- branch : HEAD

Mailbox opening and closing APIs changed. --HG-- branch : HEAD

mailbox_save_init() API was split to multiple functions. This allows adding more parameters easily in future. --HG-- branch : HEAD

mailbox.save_init() API now takes ** pointer to dest_mail, so multiple plugins can safely specify it. ctx->dest_mail also gets updated then. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Changed *_transaction_created() to static. --HG-- branch : HEAD

Removed unused macros --HG-- branch : HEAD

Removed struct index_storage abstraction. It's pointless. --HG-- branch : HEAD

Added a simple cydir mail storage backend. It trusts index files completely: if the indexes are gone, the mailbox starts completely empty and overwriting the existing mail files. So probably not a good idea to use this in production yet. --HG-- branch : HEAD