plugin/file: make non-existent file non-fatal (#2955)

* plugin/file: make non-existent file non-fatal

If the zone file being loaded doesn't exist *and* reload is enabled,
just wait the file to pop up in the normal Reload routine.

If reload is set to 0s; we keep this a fatal error on startup. Aslo fix
the ticker in z.Reload(): remove the per second ticks and just use the
reload interval for the ticker.

Brush up the documentation a bit as well.

Fixes: #2951

Signed-off-by: Miek Gieben <miek@miek.nl>

* Stickler and test compile

Signed-off-by: Miek Gieben <miek@miek.nl>

* Remove there too

Signed-off-by: Miek Gieben <miek@miek.nl>

* Cant README test these because zone files dont exist

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/file/zone.go

@@ -30,7 +30,6 @@ type Zone struct {
 	Expired      *bool
 
 	ReloadInterval time.Duration
-	LastReloaded   time.Time
 	reloadMu       sync.RWMutex
 	reloadShutdown chan bool
 	Upstream       *upstream.Upstream // Upstream for looking up external names during the resolution process.
@@ -53,7 +52,6 @@ func NewZone(name, file string) *Zone {
 		Tree:           &tree.Tree{},
 		Expired:        new(bool),
 		reloadShutdown: make(chan bool),
-		LastReloaded:   time.Now(),
 	}
 	*z.Expired = false
 	return z