Add a cachebuster to initial /sync

... in the hope of fending off weird firefox restore issues

lib/sync.js

@@ -461,9 +461,17 @@ SyncApi.prototype._sync = function(syncOptions) {
     var qps = {
         filter: filterId,
         timeout: this.opts.pollTimeout,
-        since: syncToken || undefined // do not send 'null'
     };
 
+    if (syncToken) {
+        qps.since = syncToken;
+    } else {
+        // use a cachebuster for initialsyncs, to make sure that
+        // we don't get a stale sync
+        // (https://github.com/vector-im/vector-web/issues/1354)
+        qps._cacheBuster = Date.now();
+    }
+
     if (self._syncConnectionLost) {
         // we think the connection is dead. If it comes back up, we won't know
         // about it till /sync returns. If the timeout= is high, this could