Buffering/batching client messages results in out of order events

https://github.com/karma-runner/karma/commit/c4ad69709103110a066ae1d9652af69e42434c6b introduced batching to overcome a Safari issue with xhr polling; newer Safari versions do not seem to reproduce this issue anymore.

However, this logic sometimes makes client event messages get to the server out of order. It seems to happen when socket.io upgrades from xhr polling to websockets.