[#1621] treesvc: Cancel background sync on failure

If applyOperationStream() exits prematurely, other goroutines will block
on send and errgroup will never finish waiting. In this commit we also
check whether context is cancelled.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/services/tree/sync_test.go

@@ -1,6 +1,7 @@
 package tree
 
 import (
+	"context"
 	"testing"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/pilorama"
@@ -64,7 +65,7 @@ func Test_mergeOperationStreams(t *testing.T) {
 			merged := make(chan *pilorama.Move, 1)
 			min := make(chan uint64)
 			go func() {
-				min <- mergeOperationStreams(nodeOpChans, merged)
+				min <- mergeOperationStreams(context.Background(), nodeOpChans, merged)
 			}()
 
 			var res []uint64