UserPrincipalProvider may cause many conflicts when under load

UserPrincipalProvider can be configured to periodically cache group membership by writing group principals on a rep:cache node. This will result in thundering herd problem when the system is under load and the expiration time for the cache is reached. Incoming requests that authenticate concurrently will all try to refresh the cache and cause conflicts because each request tries to set a new expiration time that is slightly different from the others.