plugin/{file,auto}: drop fsnotify (#1090)

* plugin/{file,auto}: drop fsnotify

Reload every minute. This is more deterministic then fsnotify. Also
other thing cropped up: sharing zone files between zone; there is only
1 fsnotify event and we need to fan out the reload to all zone files.
This is a large rewrite (which could still be done), for now, poll the
zone file on disk.

Give serial no change a special error type so we can check for this.
Improve the logging for reloading:

2017/09/19 07:34:39 [INFO] Successfully reloaded zone "miek.nl." in "db.miek.nl" with serial 128263060
2017/09/19 07:34:45 [INFO] Successfully reloaded zone "miek.nl." in "db.miek.nl" with serial 128263059
2017/09/19 07:34:51 [INFO] Successfully reloaded zone "miek.nl." in "db.miek.nl" with serial 128263060

Fixes #1013

* typo

plugin/file/reload_test.go

@@ -31,7 +31,9 @@ func TestZoneReload(t *testing.T) {
 		t.Fatalf("failed to parse zone: %s", err)
 	}
 
+	TickTime = 500 * time.Millisecond
 	z.Reload()
+	time.Sleep(time.Second)
 
 	r := new(dns.Msg)
 	r.SetQuestion("miek.nl", dns.TypeSOA)