AthContainersInterfaces,etc: Change getSelectedAuxIDs to return by value.

getSelectedAuxIDs() was returning a value by const reference, which was problematic for thread-safety, since that requires saving a copy of the set in the AuxSelector object. Change to returning by value. Now that auxid_set_t is represented by a bitmap with a move operator, that should not be very expensive.

Also change AuxDiscoverySvc::getauxIDs() to return by value.