Parallel push

Here is how it works:

Update every JSON one after another. (This will take care of the cookie)
Then
Update all layers at the same time.

It uses the same display as in the pull.


I experienced some races with `http.Flusher`, that's why I added a Close method the the WriteFlusher, that keep the lock to prevent anymore flush.

@creack @crosbymichael Could you take a look, its working but not fully tested though, please wait before merging.