Interrupt /sync backoff when keep-alive succeeds in order to immediately retry if we were waiting

lib/client.js

@@ -279,7 +279,7 @@ MatrixClient.prototype.retryImmediately = function() {
     // stop waiting
     clearTimeout(this._syncingRetry.timeoutId);
     // invoke immediately
-    this._syncingRetry.fn();
+    this._syncingRetry.fn(2); // FIXME: It shouldn't need to know about attempts :/
     this._syncingRetry = null;
     return true;
 };

lib/sync.js

@@ -545,6 +545,10 @@ SyncApi.prototype._sync = function(syncOptions, attempt) {
                     debuglog("Aborting current /sync.");
                     self._currentSyncRequest.abort();
                 }
+                // immediately retry if we were waiting
+                debuglog(
+                    "Interrupted /sync backoff: %s", self.client.retryImmediately()
+                );
             });
         }
         console.error("/sync error (%s attempts): %s", attempt, err);