StoreGate: remove Updater mode from VarHandleKey
The Updater mode of DataHandle is removed in Gaudi v36r1 and our
UpdateHandle uses the Reader mode anyway (see discussion in
gaudi/Gaudi!1237 (closed)). Remove usage of this mode and update unit tests. Fixes build failures in Gaudi nightlies.
Also update pythonRepr to use the now available className()
function instead of parsing the full key name.