Change private properties to protected in ClientImpl

[alexmnv]

Why make the properties of ClientImpl private? For example, I need to get an underlying socket to use it in reactphp's event-loop. Given that socket property is private and there's no getSocket function, I can't get it in any other way than modifying the source code of ClientImpl directly (which isn't good practice). If socket were protected, I could just extend the class to implement getSocket function.

[alexmnv]

Thought it would be better just to implement getSocket() method, so I created a pull request #169