Fixes COMETD-489 (Support client-side message acknowledgment).

Implemented also the changes to the Java API.
Deprecated and removed usages of ServerChannel.publish(Session, Object, String)
in favor of ServerChannel.publish(Session, Object) in line with CometD 3.
Fixed Dojo compatibility layer to invoke unsubscribe() with all parameters.
Merged branch '2.8.x' into 'master'.