Allow new transaction method in postgresql (#1493)

kv-extreme-scale needs fixing, unrelated.

lib/datasource.js

@@ -2229,6 +2229,11 @@ DataSource.prototype.transaction = function(execute, options, cb) {
       // retrieved again in determineOptions() in dao.js, as well as referenced
       // in transaction.commit() and transaction.rollback() below.
       transaction.currentTransaction = tx;
+
+      // Some connectors like Postgresql expose loobpack-connector as a property on the tx
+      if (!tx.observe && tx.connector) {
+        tx = tx.connector;
+      }
       // Handle timeout and pass it on as an error.
       tx.observe('timeout', function(context, next) {
         const err = new Error(g.f('Transaction is rolled back due to timeout'));