Restrict the POI category backfill to searchable categories

Restrict the POI category backfill to searchable categories

Only class/type combinations that have a place_classtype_* table can be
reached through a special phrase, and those are exactly the ones master
resolved through Connection.get_class_table(). Look the tables up the
same way and restrict the UPDATE to rows that match one. That takes the
planet from 303,652,952 to 46,222,253 rows and skips the step entirely
on databases that never imported special phrases.

The value expression now matches backfill_categories, so a POI also
picks up the osm.place.* category from its extratags.

Categories without a table are no longer found by the [key=value]
syntax on a migrated database. Those need a reimport.

Also unskip the [club=scout] search test. It asserted on the reported
class/type, which is the row's own main tag rather than the queried
category. Check for the two deterministic objects instead; the third
result is subject to deduplication.