Getting Xref for cross-posted messages is an O(n) operation
where `n' is the number of newsgroups on the server. This works
acceptably when there are dozens of groups, but would be
unnacceptable when there's tens of thousands of newsgroups.
With ~140 newsgroups, a lore.kernel.org mirror already handles
"XHDR Xref $MESSAGE_ID" requests around 30% faster after
creating the xref3.idx_nntp index.
The SQL additions to ExtSearch.pm may be a bit strange and
seem more appropriate for Over.pm; however it currently makes
sense to me since those bits of over.sqlite3 access are
exclusive to ExtSearch and can't be used by traditional
v1/v2 inboxes...