Support a more complete rollback of records

[forgetboy]

I would propose we do something along these lines to provide a more comprehensive rollback -

rollback(record) {
           if (!record) {
        return;
    }

    record.eachRelationship((name, descriptor) => {
        if (descriptor.kind === 'hasMany') {
            let hasManyRecords = record.get(name);
            let hasManyRecordsSize = hasManyRecords.length;
            for (let i = hasManyRecordsSize - 1; i >= 0; i--) {
                this.rollback(hasManyRecords.objectAt(i));
            }
        } else if (descriptor.kind === 'belongsTo') {
            this.rollback(record.get(name));
        }
    });

    record.rollbackAttributes();
}

[wecc]

I'm closing this as we're tracking dirtyness/rollback of relationships in emberjs/rfcs#21