middleware/cache: split cache in positive and negative and use lru (#298)

Make the cache memory bounded, by using a LRU cache. Also split the
cache in a positive and negative one - each with its own controls.

Extend the cache stanza to allow for this:

    cache {
       positive limit [ttl]
       negative limit [ttl]
    }

is now possible. This also add a cache_test.go in the toplevel test/
directory that exercises the caching path.

Fixes #260

middleware/pkg/response/classify.go

@@ -18,6 +18,22 @@ const (
 	OtherError
 )
 
+func (t Type) String() string {
+	switch t {
+	case Success:
+		return "NOERROR"
+	case NameError:
+		return "NXDOMAIN"
+	case NoData:
+		return "NODATA"
+	case Delegation:
+		return "DELEGATION"
+	case OtherError:
+		return "OTHERERROR"
+	}
+	return ""
+}
+
 // Classify classifies a message, it returns the Type.
 func Classify(m *dns.Msg) (Type, *dns.OPT) {
 	opt := m.IsEdns0()