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/algorithms/base.js

@@ -51,6 +51,7 @@ module.exports.DECRYPTION_CLASSES = {};
  * @param {module:crypto/OlmDevice} params.olmDevice olm.js wrapper
  * @param {module:base-apis~MatrixBaseApis} baseApis base matrix api interface
  * @param {string} params.roomId  The ID of the room we will be sending to
+ * @param {object} params.config  The body of the m.room.encryption event
  */
 var EncryptionAlgorithm = function(params) {
     this._userId = params.userId;