If this value is true, use (if possible) a timestamp based on message
Date or Delivery-date headers. The current system time is used
otherwise.
filename_use_mail_timestamp and utime_from_header are now completely
separated option that do not interfere one with other.
To handle this feature in a multithread context we use a hash to count
the number of mail with the same timestamp. This method is more accurate
than using the old lasttime and timeseq variables.
Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>