for "fresh" messages, calc_sort_timestamp() makes sure,
the sorting timestamp is not less than the last-non-fresh message.
this commit fixes the "fresh" state we give to calc_sort_timestamp(),
it just uses the message-state we've already calculated.
the old assumption, that unseen messages are always fresh
is wrong as this "seen" flag just comes from an imap-flag
that is not set for self-sent messages.
therefore, in a multi-device-setup, things are totally messed up.
(the bug was probably in there since a long time,
however did not came to light until the async-move)