Adds create order transactiom

packages/medusa-interfaces/src/base-model.js

@@ -44,6 +44,13 @@ class BaseModel {
     return mongoose.model(this.getModelName(), this.getSchema())
   }
 
+  /**
+   * @private
+   */
+  startSession() {
+    return this.mongooseModel_.startSession()
+  }
+
   /**
    * Queries the mongoose model via the mongoose's findOne.
    * @param query {object} a mongoose selector query