Updated benchmark server to use Jetty 9.1 WebSocket configuration.
Rewritten the AsyncLongPollingTransport implementation following GregW suggestions.
Updated test to use Jetty-specific WebSocket configuration,
rather than JSR 356 Jetty-specific WebSocket configuration.
Fixes COMETD-458 (Seti: Duplicate messages, isPresent inconsistencies
 after multiple node crashes/reassociations).

Fixed a race condition where a presence message could have been
processed just after a "comet left" event, thus leaving the Seti with
 an association belonging to a Seti that does not exist anymore.
Now Seti uses a mechanism similar to OortObject to push its state to
a peer, and disassociates users if the node they have been associated
 is not connected anymore.
Converting infinite wait into a timed wait, and guarding with a try/finally code inside the CompletionListener to be sure the Future is notified.
Merged branch '2.7.x' into 'master'.