The Identifier of clause that defines a Rx session repository.
For example, the SQL based RxSession module shipped with Radiator policy
and charging support modules is configured with
<PCRFRxSessionSQL> clause.
The RxSession
clause must implement the interface defined by the
PCRFRxSessionGeneric.pm module. The backend is not
limited to SQL but should use a a backend that supports frequent creation
and updating of session data