Should SFrameTransform receiver be made aware of the current key in use, so that it would call removeEncryptionKey appropriately?

A web app might want to remove SFrame keys on receiver side whenever it can (good for memory) but it might not know when it should do so.
Currently, it does not know when the SFrame receiver transform is using a new key, so that it can remove the old key. And there might be cases where SFrames are decrypted out of order as well.
