[#1462] writecache: Fill flush marks during startup

Some of the objects are already flushed, don't do it twice.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/local_object_storage/writecache/writecache.go

@@ -133,6 +133,8 @@ func (c *cache) Open() error {
 
 // Init runs necessary services.
 func (c *cache) Init() error {
+	c.initFlushMarks()
+
 	go c.persistLoop()
 	go c.flushLoop()
 	return nil