Add setSurvey() utility function to DB interface.

It sets the current survey and optionally returns a
client connection to use in subsequent queries.

lib/www/server/lib/db/connection.js

@@ -28,7 +28,16 @@ const transaction = {
 	}
 };
 
+async function setSurvey (projectId, client) {
+	if (!client) {
+		client = await pool.connect();
+	}
+	await client.query("CALL set_survey($1);", [projectId]);
+	return client;
+}
+
 module.exports = {
 	pool,
 	transaction,
+	setSurvey
 };