Cache (#126)

* Add middleware/cache

Add a caching middleware that caches nxdomain, nodata and successful
responses. It differentiates between DNSSEC on normal DNS replies.

Each reply is compress and scrubbed so it will fit the specific client
asking for it.

* first simple test, less exporting of stuff

* more

* Add middleware/cache

Add a caching middleware that caches nxdomain, nodata and successful
responses. It differentiates between DNSSEC on normal DNS replies.

Each reply is compressed and scrubbed so it will fit the specific client
asking for it. The TTL is decremented with the time spend in the cache.
There is syntax that allows you to cap the TTL for all records, no
matter what. This allows for a shortlived cache, just to absorb query
peaks.

+Tests

* cache test infrastructure

* Testing

middleware/cache/item_test.go

@@ -0,0 +1,25 @@
+package cache
+
+import (
+	"testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestKey(t *testing.T) {
+	if noDataKey("miek.nl.", dns.TypeMX, false) != "0miek.nl...15" {
+		t.Errorf("failed to create correct key")
+	}
+	if noDataKey("miek.nl.", dns.TypeMX, true) != "1miek.nl...15" {
+		t.Errorf("failed to create correct key")
+	}
+	if nameErrorKey("miek.nl.", false) != "0miek.nl." {
+		t.Errorf("failed to create correct key")
+	}
+	if nameErrorKey("miek.nl.", true) != "1miek.nl." {
+		t.Errorf("failed to create correct key")
+	}
+	if noDataKey("miek.nl.", dns.TypeMX, false) != successKey("miek.nl.", dns.TypeMX, false) {
+		t.Errorf("nameErrorKey and successKey should be the same")
+	}
+}