AckExtension
This client-side extension enables the client to acknowledge to the server
the messages that the client has received.
For the acknowledgement to work, the server must be configured with the
correspondent server-side ack extension. If both client and server support
the ack extension, then the ack functionality will take place automatically.
By enabling this extension, all messages arriving from the server will arrive
via the long poll, so the comet communication will be slightly chattier.
The fact that all messages will return via long poll means also that the
messages will arrive with total order, which is not guaranteed if messages
can arrive via both long poll and normal response.
Messages are not acknowledged one by one, but instead a group of messages is
acknowledged when long poll returns.