Restart broken Olm sessions

* Start a new Olm sessions with a device when we get an undecryptable
   message on it.
 * Send a dummy message on that sessions such that the other end knows
   about it.
 * Re-send any outstanding keyshare requests for that device.

Also includes a unit test for megolm that isn't very related but came
out as a result anyway.

Includes https://github.com/matrix-org/matrix-js-sdk/pull/776
Fixes https://github.com/vector-im/riot-web/issues/3822

src/crypto/OutgoingRoomKeyRequestManager.js

@@ -244,6 +244,21 @@ export default class OutgoingRoomKeyRequestManager {
         });
     }
 
+    /**
+     * Look for room key requests by target device and state
+     *
+     * @param {string} userId Target user ID
+     * @param {string} deviceId Target device ID
+     *
+     * @return {Promise} resolves to a list of all the
+     *    {@link module:crypto/store/base~OutgoingRoomKeyRequest}
+     */
+    getOutgoingSentRoomKeyRequest(userId, deviceId) {
+        return this._cryptoStore.getOutgoingRoomKeyRequestsByTarget(
+            userId, deviceId, [ROOM_KEY_REQUEST_STATES.SENT],
+        );
+    }
+
     // start the background timer to send queued requests, if the timer isn't
     // already running
     _startTimer() {