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.

src/client.js

@@ -5201,6 +5201,8 @@ module.exports.CRYPTO_ENABLED = CRYPTO_ENABLED;
  * @param {object} data
  * @param {MatrixEvent} data.event the original verification request message
  * @param {Array} data.methods the verification methods that can be used
+ * @param {Number} data.timeout the amount of milliseconds that should be waited
+ *                 before cancelling the request automatically.
  * @param {Function} data.beginKeyVerification a function to call if a key
  *     verification should be performed.  The function takes one argument: the
  *     name of the key verification method (taken from data.methods) to use.