add checksum based status check, alternative to timestamp based

status/status.go

@@ -0,0 +1,13 @@
+package status
+
+var (
+	_ Checker = &Timestamp{}
+	_ Checker = &Checksum{}
+	_ Checker = None{}
+)
+
+// Checker is an interface that checks if the status is up-to-date
+type Checker interface {
+	IsUpToDate() (bool, error)
+	OnError() error
+}