[#255] write-cache: Fix init race condition

Do not use WC's internals in the initialization routines without mode
protection. WC should be able to change its mode even if the initialization
is not finished yet.

Signed-off-by: Pavel Karpy <p.karpy@yadro.com>

pkg/local_object_storage/writecache/init.go

@@ -86,13 +86,21 @@ func (c *cache) fsTreeFlushMarkUpdate(ctx context.Context) {
 		}
 		return nil
 	}
+
+	c.modeMtx.RLock()
+	defer c.modeMtx.RUnlock()
+
 	_, _ = c.fsTree.Iterate(prm)
+
 	c.log.Info(logs.WritecacheFinishedUpdatingFSTreeFlushMarks)
 }
 
 func (c *cache) dbFlushMarkUpdate(ctx context.Context) {
 	c.log.Info(logs.WritecacheFillingFlushMarksForObjectsInDatabase)
 
+	c.modeMtx.RLock()
+	defer c.modeMtx.RUnlock()
+
 	var m []string
 	var indices []int
 	var lastKey []byte