use adapter for to_device requests to have same api as for verif over DM
Riot doesn't fully implement to_device verifications, e.g.
it doesn't send a `request` but immediately sends a `start` event.
Because of this, `crypto.verification.request` doesn't get fired,
as that code path doesn't get triggered. This is why MatrixChat
in the react-sdk was listening for `crypto.verification.start`.
Verification over DM *does* send a `request` event first, so
to have the same API for both methods, we fake the request and
wrap the verifier in it.