Lock down the ability to set/reset EJB client context selector on the server side by user code

On the server side we shouldn't allow setting/resetting the EJB client context selector, by the user code. An API to lock down setting the selector is being added in the next version of EJB client API. The server should use that API to lock down the selector once it has setup the server wide selector.

Creating this as a task for 7.1.0.CR1 so as to not forget adding this implementation.