Previously we lowercased names on ingress but not on lookup or delete
which could cause unexpected results. Now, just unilaterally lowercase
policy names on write and delete. On get, to avoid the performance hit
of always lowercasing when not necessary since it's in the critical
path, we have a minor optimization -- we check the LRU first before
normalizing. For tokens, because they're already normalized when adding
policies during creation, this should always work; it might just be
slower for API calls.
Fixes#3187
Jeff Mitchell