what's new in v2.0

[wsky]

• more clear design layer

connector(I/O)    |    top-push             |   backend
onConnect      ----->  addConnection          
onDisconnect  -----> removeConnection
                       send  <------------------ send

• message ordering
• new messaging schduler design
• new message sender
• more api for more cases

// async send
client.send(msg, ordering, callback);
// for batch and send immediately
Sender sender = client.createSender();
sender.send();