middleware/file: don't reload zone when SOA isn't changed (#707)

* middleware/file: don't reload zone when SOA isn't changed

Give Parse an extra argument which is the SOA's serial, if > 0 we check
against the just parsed SOA and then just return.

Most notable use is in reload.go which is both used in the file and auto
middleware.

Fixes #415

* PR comments

middleware/file/glue_test.go

@@ -34,7 +34,7 @@ var atoomTestCases = []test.Case{
 }
 
 func TestLookupGlue(t *testing.T) {
-	zone, err := Parse(strings.NewReader(dbAtoomNetSigned), atoom, "stdin")
+	zone, err := Parse(strings.NewReader(dbAtoomNetSigned), atoom, "stdin", 0)
 	if err != nil {
 		t.Fatalf("Expected no error when reading zone, got %q", err)
 	}