Cache elements of State

Cache the size and the do bit whenever someone asked for it. We can

probably add more:



PASS

BenchmarkStateDo-4  	100000000	        11.9 ns/op

BenchmarkStateSize-4	 5000000	       265 ns/op

ok  	github.com/miekg/coredns/middleware	2.828s



PASS

BenchmarkStateDo-4  	1000000000	         2.86 ns/op

BenchmarkStateSize-4	500000000	         3.10 ns/op

ok  	github.com/miekg/coredns/middleware	5.032s



This PR also includes some testing cleanups as well.

middleware/state_test.go

@@ -1,5 +1,46 @@
 package middleware
 
+import (
+	"testing"
+
+	coretest "github.com/miekg/coredns/middleware/testing"
+
+	"github.com/miekg/dns"
+)
+
+func TestStateDo(t *testing.T) {
+	st := testState()
+
+	st.Do()
+	if st.do == 0 {
+		t.Fatalf("expected st.do to be set")
+	}
+}
+
+func BenchmarkStateDo(b *testing.B) {
+	st := testState()
+
+	for i := 0; i < b.N; i++ {
+		st.Do()
+	}
+}
+
+func BenchmarkStateSize(b *testing.B) {
+	st := testState()
+
+	for i := 0; i < b.N; i++ {
+		st.Size()
+	}
+}
+
+func testState() State {
+	m := new(dns.Msg)
+	m.SetQuestion("example.com.", dns.TypeA)
+	m.SetEdns0(4097, true)
+
+	return State{W: &coretest.ResponseWriter{}, Req: m}
+}
+
 /*
 func TestHeader(t *testing.T) {
 	state := getContextOrFail(t)