Persist txnId to ensure idempotency

Author: germain-gg

src/models/event.js

@@ -168,7 +168,7 @@ export const MatrixEvent = function(
     /* The txnId with which this event was sent if it was during this session,
        allows for a unique ID which does not change when the event comes back down sync.
      */
-    this._txnId = null;
+    this._txnId = event.txn_id || null;
 
     /* Set an approximate timestamp for the event relative the local clock.
      * This will inherently be approximate because it doesn't take into account
@@ -1098,6 +1098,7 @@ utils.extend(MatrixEvent.prototype, {
             origin_server_ts: this.getTs(),
             unsigned: this.getUnsigned(),
             room_id: this.getRoomId(),
+            txn_id: this.getTxnId(),
         };
 
         // if this is a redaction then attach the redacts key

src/models/room.js

@@ -195,11 +195,9 @@ export function Room(roomId, client, myUserId, opts) {
         if (serializedPendingEventList) {
             JSON.parse(serializedPendingEventList)
                 .forEach(serializedEvent => {
-                    const txnId = client.makeTxnId();
                     const event = new MatrixEvent(serializedEvent);
                     event.setStatus(EventStatus.NOT_SENT);
-                    event.setTxnId(txnId);
-                    this.addPendingEvent(event, txnId);
+                    this.addPendingEvent(event, event.getTxnId());
                 });
         }
     }