FIX: Reconnect in restore process connects to correct DB (#8218)

Simplified flow of restore is like that
```
migrate_database
reconnect
extract_uploads
```

Problem with incorrect current database started with this fix https://github.com/discourse/discourse/commit/025d4ee91f4

Dump task is reconnecting to default database https://github.com/rails/rails/blob/master/activerecord/lib/active_record/railties/databases.rake#L429

And then, we are trying to reconnect to the original database with that code:
```
def reconnect_database
  log "Reconnecting to the database..."
  RailsMultisite::ConnectionManagement::establish_connection(db: @current_db)
end
```

This reconnect is not switching us back to correct database because of that check
https://github.com/discourse/rails_multisite/blob/master/lib/rails_multisite/connection_management.rb#L181
Basically, it finds existing handler and it thinks that we are connected to correct DB and this step can be skipped.

To solve it, we can reload RailsMultisite::ConnectionManagement which creates a new instance of that class
https://github.com/discourse/rails_multisite/blob/master/lib/rails_multisite/connection_management.rb#L38

lib/backup_restore/restorer.rb

@@ -404,6 +404,7 @@ module BackupRestore
 
     def reconnect_database
       log "Reconnecting to the database..."
+      RailsMultisite::ConnectionManagement::reload
       RailsMultisite::ConnectionManagement::establish_connection(db: @current_db)
     end
 

spec/lib/backup_restore/restorer_spec.rb

@@ -100,4 +100,26 @@ describe BackupRestore::Restorer do
       @restorer.copy_archive_to_tmp_directory
     end
   end
+
+  context 'Database connection' do
+    fab!(:admin) { Fabricate(:admin) }
+    before do
+      SiteSetting.allow_restore = true
+      @restore_path = File.join(Rails.root, 'public', 'backups', RailsMultisite::ConnectionManagement.current_db)
+      described_class.any_instance.stubs(ensure_we_have_a_filename: true)
+      described_class.any_instance.stubs(initialize_state: true)
+    end
+    let(:conn) { RailsMultisite::ConnectionManagement }
+    let(:restorer) { described_class.new(admin.id) }
+
+    it 'correctly reconnects to database' do
+      restorer.instance_variable_set(:@current_db, 'second')
+      conn.config_filename = "spec/fixtures/multisite/two_dbs.yml"
+      conn.establish_connection(db: 'second')
+      expect(RailsMultisite::ConnectionManagement.current_db).to eq('second')
+      ActiveRecord::Base.connection_pool.spec.config[:db_key] = "incorrect_db"
+      restorer.send(:reconnect_database)
+      expect(RailsMultisite::ConnectionManagement.current_db).to eq('second')
+    end
+  end
 end