Orders should not implement EntityOwnerInterface

EntityOwnerInterface says:

/**
* Defines a common interface for entities that have an owner.
*
* An owner is someone who has primary control over an entity, similar to
* owners in Unix file system access. This may or may not be the entity's
* original author. The owner may also have less permissions than other users,
* such as administrators.
*/

The user to which the order is assigned does not have primary control over an entity.
The merchant is the one that handles order processing. These wrong semantics can be seen in the permissions, where we have "create own orders", "update own orders", "delete own orders".

So, we need to stop extending EntityOwnerInterface from OrderInterface, and we need to rename the owner methods to customer methods (getOwnerId() -> getCustomerId(), etc). We also need a change record, since we're in beta.