extsearch: unconditionally reopen on access

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.

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