Refactor decryption

Create the MatrixEvent wrapper before decryption, and then pass that into the
decryptors, which should update it.

Also remove the workaround that sends m.new_device messages when we get an
unknown session; it's just a bandaid which is obscuring more meaningful
problems.

lib/crypto/algorithms/megolm.js

@@ -451,7 +451,7 @@ utils.inherits(MegolmDecryption, base.DecryptionAlgorithm);
 /**
  * @inheritdoc
  *
- * @param {object} event raw event
+ * @param {MatrixEvent} event
  *
  * @return {null} The event referred to an unknown megolm session
  * @return {module:crypto.DecryptionResult} decryption result
@@ -460,7 +460,7 @@ utils.inherits(MegolmDecryption, base.DecryptionAlgorithm);
  *   problem decrypting the event
  */
 MegolmDecryption.prototype.decryptEvent = function(event) {
-    var content = event.content;
+    var content = event.getWireContent();
 
     if (!content.sender_key || !content.session_id ||
         !content.ciphertext
@@ -471,14 +471,15 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
     var res;
     try {
         res = this._olmDevice.decryptGroupMessage(
-            event.room_id, content.sender_key, content.session_id, content.ciphertext
+            event.getRoomId(), content.sender_key, content.session_id, content.ciphertext
         );
     } catch (e) {
         throw new base.DecryptionError(e);
     }
 
     if (res === null) {
-        return null;
+        // We've got a message for a session we don't have.
+        throw new base.DecryptionError("Unknown inbound session id");
     }
 
     var payload = JSON.parse(res.result);
@@ -486,17 +487,13 @@ MegolmDecryption.prototype.decryptEvent = function(event) {
     // belt-and-braces check that the room id matches that indicated by the HS
     // (this is somewhat redundant, since the megolm session is scoped to the
     // room, so neither the sender nor a MITM can lie about the room_id).
-    if (payload.room_id !== event.room_id) {
+    if (payload.room_id !== event.getRoomId()) {
         throw new base.DecryptionError(
             "Message intended for room " + payload.room_id
         );
     }
 
-    return {
-        payload: payload,
-        keysClaimed: res.keysClaimed,
-        keysProved: res.keysProved,
-    };
+    event.setClearData(payload, res.keysClaimed, res.keysProved);
 };
 
 /**