net/http/authz: use stale read in Delete

[jbowens]

The Delete method does a read-modify-write so the read doesn't need to be linearizable. If it reads a stale value, the IfNotModified condition will fail the sinkdb operation.

By avoiding a consensus round on every call to Delete we also bring down deleteGrantsByAccessToken's # of consensus rounds from n + 1 to 1, where n is the number of policies.

[tessr]

👀

[tessr]

LGTM