middleware/cache: rename categories (#321)

Rename: positive -> success
negative -> denial

There is a third (unused category) which is error. Start using these
new in the caching middleware and later in the logging middleware.

middleware/cache/cache.go

@@ -15,7 +15,7 @@ import (
 )
 
 // Cache is middleware that looks up responses in a cache and caches replies.
-// It has a positive and a negative cache.
+// It has a success and a denial of existence cache.
 type Cache struct {
 	Next  middleware.Handler
 	Zones []string