wc_ delimiter to identify start of uniquid

I need to determine if an _order_key was generated by WooCommerce, or if it is unique. A delimiter prefixing the _order_key, like wc_ for example, would allow this determination to always be correct.

Another possibility might be to simply use order_ as the prefix delimiter:

```php
'order_' . apply_filters('woocommerce_generate_order_key', uniqid() ) );
```
I personally like the wc_ prefix better, since it differentiates a WooCommerce order key value from other shopping cart order key values that may exist with the same _order_key post meta key.