[#1298] writecache: Add shrink flag for Seal command

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

pkg/local_object_storage/writecache/writecache.go

@@ -23,6 +23,7 @@ type Info struct {
 type SealPrm struct {
 	IgnoreErrors bool
 	RestoreMode  bool
+	Shrink       bool
 }
 
 // Cache represents write-cache for objects.