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/file_test.go

@@ -7,6 +7,6 @@ import (
 
 func BenchmarkParseInsert(b *testing.B) {
 	for i := 0; i < b.N; i++ {
-		Parse(strings.NewReader(dbMiekENTNL), testzone, "stdin")
+		Parse(strings.NewReader(dbMiekENTNL), testzone, "stdin", 0)
 	}
 }