by aaronbauman - update hook_entity operations for salesforce_push for D8

Since push operations are handled inside MappedObject, we don't need all this logic inside salesforce_push. My general goal here is to get real-time REST operations up and running, then move on to queue/batch operations, and leverage SOAP where appropriate. End users shouldn't have to decide whether to use REST or SOAP or even know the difference. Drupal should use them as appropriate, given admin settings. To that end, salesforce_push_cron is still just a stub and hasn't been touched. MappedObject is way too fat, and will probably be slimmed down in the future - especially push() and pull(), which belong somewhere else.