Logout answered an undeclared 404 when its grant was already gone, for
example revoked by another node inside the liveness cache window or by a
concurrent logout. It now treats a missing grant as success and still
evicts the cache entry, so logout always answers 200.
Grants left on an older user epoch (after a password reset through the
existing UI, or a login that raced a password change) are dead but only
deleted when presented. Listing and counting grants now filter on the
user's current epoch, so those grants no longer show up.
dropGrant now deletes before evicting, like RevokeGrant, so a concurrent
cache fill cannot re-cache a grant that is being dropped.
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