Associate a context with request

Since all requests issued to Jzab is asynchronous, we need a way to match the completed requests in callback with the one we issued. 

Associating a context object with each requests seems to be a good way to solve this, like what ZooKeeper does.

```
void send(ByteBuffer req, Object ctx);
void flush(ByteBuffer req, Object ctx);
void remove(String serverId, Object ctx);
```