Rename FieldConfig to and FieldInstance

Follow-up to #2226197: Introduce FieldStorageDefinitionInterface in the Entity Field API: Once we've got a StorageFieldDefinitionInterface, it would make sense to rename FieldConfig to StorageConfig, just as the CMI name and the interface. Similarly, we could rename FieldInstance to FieldConfig then, as it's the real, fully usable field definition. As noted in #2226197: Introduce FieldStorageDefinitionInterface in the Entity Field API renaming field instances has far reaching consequences for the terminology used in documentation, etc. - so we might prefer to stay with field instances instead.

So let's discuss here whether and what to we want to rename those classes (once again).