Currently, requests that were successfully passed through the metainfo
endpoints rate-limiter might still fail in the middle of the
corresponding response. The problem is that we perform rate-limiting a
second time, which means other requests would influence whether the
current (already rate-checked) request will fail. This also has other
unintended effects, like responding with rpcstatus.PermissionDenied for
requests that were successfully rate-checked and did not lack
permissions but were rate-checked again in the middle of
(*Endpoint).BeginObject. This situation has been happening on the
gateway side and might affect other uplink clients. This change, where
appropriate, swaps subsequent validateAuth with validateAuthN that
performs rate-limiting once.
Change-Id: I6fc26dedb8c442dd20acaab5942f751279020b08
