Prohibit client use when a shared client wraps it

[ungive]

Currently shared clients can use a client that is already in use. The documentation says not to do this, but it's not enforced by runtime checks. It would be good to make the SharedClient class a friend class of Client or ClientImpl and add checks that the client isn't "dirty", when creating a SharedClient. In the same way the Client itself should throw an exception as soon as any of its methods are used that may only be used by the SharedClient, when a SharedClient instance exists that wraps the client.