After a successful verification with ourselves, request the key
backup key too and restore a key backup if we get it.
Also cache the key backup key when we cache the SSK & USK so we have
it available to share.
Fixes https://github.com/vector-im/riot-web/issues/12704
Rather than accessing private method to get the secret storage
object (this was a bit confusing when I grepped for 'requestSecret'
and didn't find anything).
* For self-verifications, also request keys from the other device
* removed some XXX's so the editor doesn't think it's three issues
* add methods to access key cache callbacks