[11] Splitquote now returns multiple order ids

[nishant-lbb]

Description

The rest API endpoints for creating orders from a quote/cart return just a single order ID, instead of a list of orders created from the original cart.

Fixed Issues (if relevant)

1. magestat/magento2-split-order#11 (https://github.com/magestat/magento2-split-order/issues/11)

Manual testing scenarios

1. Add multiple items to a cart.
2. Ensure that the plugin is enabled.
3. Create an order via the REST endpoint. v1/carts/:cartID/order

Contribution checklist

• [x] Pull request has a meaningful description of its purpose
• [x] All commits are accompanied by meaningful commit messages
• [x] All new or changed code is covered with unit/integration tests (if applicable)
• [x] All automated tests passed successfully (all builds on Travis CI are green)