Associate a context with request

[EasonLiao]

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);

[EasonLiao]

Probably we don't need clientId in deliver callback anymore?

[EasonLiao]

@m1ch1

[EasonLiao]

@m1ch1

[EasonLiao]

some code refactoring on CommitProcessor