Fix PostgresModelDataMigrator leaving the swapped-in table without privileges, stats and autovacuum

[Photonios]

PostgresModelDataMigrator ran VACUUM ANALYZE and the autovacuum reset outside a transaction, where SET LOCAL search_path does nothing, so both hit the original table. The swapped-in table was left with autovacuum_enabled=false and no statistics. postgres_prepend_local_search_path now raises outside a transaction, like postgres_set_local already does.

The swapped-in table also lost all its privileges, because the default privileges of a schema don't apply to a table created in another schema. They are now copied right before the swap. Column privileges, and sequence privileges on Django 4.0 and older, are not copied.