Pass through function to discard megolm session

To make debugging crypto slightly faster

src/client.js

@@ -702,6 +702,21 @@ MatrixClient.prototype.isRoomEncrypted = function(roomId) {
     return this._roomList.isRoomEncrypted(roomId);
 };
 
+/**
+ * Forces the current outbound group session to be discarded such
+ * that another one will be created next time an event is sent.
+ *
+ * @param roomId The ID of the room to discard the session for
+ *
+ * This should not normally be necessary.
+ */
+MatrixClient.prototype.forceDiscardSession = function(roomId) {
+    if (!this._crypto) {
+        throw new Error("End-to-End encryption disabled");
+    }
+    return this._crypto.forceDiscardSession(roomId);
+};
+
 /**
  * Get a list containing all of the room keys
  *

src/crypto/algorithms/megolm.js

@@ -1,5 +1,6 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2018 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -496,6 +497,16 @@ MegolmEncryption.prototype.encryptMessage = function(room, eventType, content) {
     });
 };
 
+/**
+ * Forces the current outbound group session to be discarded such
+ * that another one will be created next time an event is sent.
+ *
+ * This should not normally be necessary.
+ */
+MegolmEncryption.prototype.forceDiscardSession = function() {
+    this._setupPromise = this._setupPromise.then(() => null);
+};
+
 /**
  * Checks the devices we're about to send to and see if any are entirely
  * unknown to the user.  If so, warn the user, and mark them as known to

src/crypto/index.js

@@ -621,6 +621,23 @@ Crypto.prototype.getEventSenderDeviceInfo = function(event) {
     return device;
 };
 
+/**
+ * Forces the current outbound group session to be discarded such
+ * that another one will be created next time an event is sent.
+ *
+ * @param roomId The ID of the room to discard the session for
+ *
+ * This should not normally be necessary.
+ */
+Crypto.prototype.forceDiscardSession = function(roomId) {
+    const alg = this._roomEncryptors[roomId];
+    if (alg === undefined) throw new Error("Room not encrypted");
+    if (alg.forceDiscardSession === undefined) {
+        throw new Error("Room encryption algorithm doesn't support session discarding");
+    }
+    alg.forceDiscardSession();
+};
+
 /**
  * Configure a room to use encryption (ie, save a flag in the sessionstore).
  *