[#1700] gc: Wait for handlers on GC stopping

First wait for goroutine handles epoch events to not to get data race
on `gc.newEpochHandlers.cancelFunc`.

Then cancel handlers and wait for them.

Change-Id: I71f11f8526961f8356f582a95b10eb8340c0aedd
Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/shard/gc.go

@@ -227,6 +227,9 @@ func (gc *gc) stop(ctx context.Context) {
 
 	gc.log.Info(ctx, logs.ShardWaitingForGCWorkersToStop)
 	gc.wg.Wait()
+
+	gc.newEpochHandlers.cancelFunc()
+	gc.newEpochHandlers.prevGroup.Wait()
 }
 
 // iterates over metabase and deletes objects