Changed payload proxy cache as required from Framework

The caching policy of the PayloadProxy has been changed according to the requirements of the Framework. When a invalidateTransientCache() is requested in the DataProxy, a corresponding new function invalidateTransientCache() is called in the PayloadProxy. This call reset the Payload ( when available) and clears its Payload ID, while the current IOV is kept for further use.  
The former function, invalidateCache(), which is resetting the entire IOV cache ( IOV+Payload), is currently not called by the Framework.