Inform the client instance of the returned protocol version

As per Connect#deserialize we get back the protocol version known by the server. With all current ZK implementations this will always be 0, though with https://issues.apache.org/jira/browse/ZOOKEEPER-1607 that might change (ditto with future protocol extensions).

So lets save the returned value in the client instance so it can be introspected. For now, a private instance var (i.e.: _protocol_version) should do. I guess later on we could make it public. Not adding tests since this is just copying around something that the ZK protocol has supported forever. 

Signed-off-by: Raul Gutierrez S <rgs@itevenworks.net>