Add pid2schema function.

Translates a project ID into a database schema name.

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

@@ -49,6 +49,15 @@ async function schema2pid (schema, client) {
 	return res.rows[0] && res.rows[0].pid;;
 }
 
+async function pid2schema (pid, client) {
+	if (!client) {
+		client = await pool.connect();
+	}
+	const res = await client.query("SELECT schema FROM projects WHERE pid = $1", [pid]);
+	client.release();
+	return res.rows[0] && res.rows[0].schema;
+}
+
 /** Fetch one row from a database cursor.
  *
  * @a cursor A query cursor
@@ -75,5 +84,6 @@ module.exports = {
 	transaction,
 	setSurvey,
 	schema2pid,
+	pid2schema,
 	fetchRow
 };