 Scheduled message model (holds post values generated by the composer to delay the
    posting of the message). Different from mail.message.schedule that posts the message but
    delays the notification process.

    Todo: when adding support for scheduling messages in mass_mail mode, could add a reference to
    the "parent" composer (by making 'mail.compose.message' not transient anymore). This reference
    could then be used to cancel every message scheduled "at the same time" (from one composer),
    and to get the static 'notification parameters' (mail_server_id, auto_delete,...) instead of
    duplicating them for each scheduled message.
    Currently as scheduling is allowed in monocomment only, we don't have duplicates and we only
    have static notification parameters, but some will become dynamic when adding mass_mail support
    such as 'email_from' and 'force_email_lang'.
    