lib-storage: Moved some items from mailbox_get_status() to a new mailbox_get_metadata().

lib-storage: Moved some items from mailbox_get_status() to a new mailbox_get_metadata().
The idea is now that all status items are tracked all the time after mailbox
is opened and they can always be looked up without failure. The metadata
items are looked up lazily and the lookups may fail at any time.

mailbox_get_status() can be used after mailbox_alloc() to indicate that the
mailbox doesn't necessarily have to be opened, just that the status fields
get returned.

If mailbox is already known to be open, mailbox_get_open_status() can be
used. It never fails.