Since ExtSearch lacks the janky cleanup timer of
PublicInbox::Inbox objects, its search results get stale.
Reopen the Xapian DB on every ->search call for now, as
reducing reopen calls doesn't seem worth the complexity.
The Xapian::Database::reopen operation itself takes only ~50us
on my old workstation with 3 shards totaling <200GB. Other
parts of Xapian dominates the search time, so the reopen seems
inconsequential with single-digit shard counts.