Changed server-side processing order: now extension's send*() happens before DeQueueListeners in WebSocket too.
Fixed abort() method to abort only once.
COMETD-520 - Batch replies of batched requests.

Now batching replies on the server and sending them as a single
WebSocket message.
Backported CometD 3 HTTP server transport in order to have the same
processing as the WebSocket transport, in particular regarding the order
of invocation between extension's send*() and DeQueueListeners.
Merged branch '2.8.x' into 'master'.