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/dnssec/handler_test.go

@@ -81,7 +81,7 @@ var dnsTestCases = []test.Case{
 }
 
 func TestLookupZone(t *testing.T) {
-	zone, err := file.Parse(strings.NewReader(dbMiekNL), "miek.nl.", "stdin")
+	zone, err := file.Parse(strings.NewReader(dbMiekNL), "miek.nl.", "stdin", 0)
 	if err != nil {
 		return
 	}