(new header properties: upvotes and downvotes), regardless of the
"include_votes" argument value (default: false).
get_all_msg_headers() reads the entire index already, so there's really no
performance penalty unless there are actual votes (and even then, very low).
The MsgBase.get_msg_header() function will *not* return these vote totals since
that would incur a pretty substatial performance hit on that function.