search: get rid of most lookup_* subroutines

search: get rid of most lookup_* subroutines

Too many similar functions doing the same basic thing was
redundant and misleading, especially since Message-ID is
no longer treated as a truly unique identifier.

For displaying threads in the HTML, this makes it clear
that we favor the primary Message-ID mapped to an NNTP
article number if a message cannot be found.