Rotate megolm sessions

In order to mitigate backward-secrecy concerns, make sure that we rotate the
outbound megolm session at regular intervals (every week/100 msgs by default).

lib/crypto/index.js

@@ -716,6 +716,7 @@ Crypto.prototype.setRoomEncryption = function(roomId, config) {
         olmDevice: this._olmDevice,
         baseApis: this._baseApis,
         roomId: roomId,
+        config: config,
     });
     this._roomAlgorithms[roomId] = alg;
 };