persist DecryptionAlgorithm instances

It's useful to be able to keep state between events in the DecryptionAlgorithm,
so store them in a map.

lib/crypto/algorithms/base.js

@@ -105,11 +105,16 @@ EncryptionAlgorithm.prototype.onNewDevice = function(userId, deviceId) {};
  *
  * @param {object} params parameters
  * @param {string} params.userId  The UserID for the local user
+ * @param {module:crypto} params.crypto crypto core
  * @param {module:crypto/OlmDevice} params.olmDevice olm.js wrapper
+ * @param {string=} params.roomId The ID of the room we will be receiving
+ *     from. Null for to-device events.
  */
 var DecryptionAlgorithm = function(params) {
     this._userId = params.userId;
+    this._crypto = params.crypto;
     this._olmDevice = params.olmDevice;
+    this._roomId = params.roomId;
 };
 /** */
 module.exports.DecryptionAlgorithm = DecryptionAlgorithm;