Unify implementation patterns in OKB service clients

[abeham]

he QueryClient uses singleton in combination with instance methods to perform service calls. The AdministrationClient uses a combination of singleton pattern and static methods to perform service calls.

To me it seems that the client provides the following functionality:

Caching some data obtained from the remote service (in the format in that it is obtained from the service) - implemented in form of a Refresh() method on a singleton instance and properties for the data. Making calls to the remote service to receive further data (that is not cached) - implemented in form of static or instance methods. The design may be improved if these cases are separated in two classes.

(Follow up to #2547)