leveldb2: support subscribing from peers

weed/filer2/filerstore.go

@@ -29,6 +29,11 @@ type FilerStore interface {
 	Shutdown()
 }
 
+type FilerLocalStore interface {
+	UpdateOffset(filer string, lastTsNs int64) error
+	ReadOffset(filer string) (lastTsNs int64, err error)
+}
+
 type FilerStoreWrapper struct {
 	actualStore FilerStore
 }