need a better lifecycle design that objects of endpointproxy can be used properly

Identity not unique for same endpoint

https://github.com/wsky/top-link/blob/master/java/src/main/java/com/taobao/top/link/endpoint/EndpointChannelHandler.java#L202

``` java
Identity id = this.endpoint.getIdentity().parse(connectMessage.content);
                        //...
                        this.idByToken.put(proxy.getToken(), id);
```

also about context or others
