Check the URL length in case of cross-domain comet

When using Comet via cross domain, the jsonp callback mechanism is used; in this case, bayeux messages are sent via GET encoded in the URL.
However, URL have a max length, and sending many bayeux messages (for example via batch) can exceed the URL max length.
At least a warning should be emitted in such case, or find a way to send the messages without exceeding the URL max length.