[#569] cli: Add control shards writecache seal command

It does the same as `control shards flush-writecache --seal`, but
has better name.

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

pkg/local_object_storage/writecache/writecache.go

@@ -36,6 +36,7 @@ type Cache interface {
 	SetLogger(*logger.Logger)
 	DumpInfo() Info
 	Flush(context.Context, bool, bool) error
+	Seal(context.Context, bool) error
 
 	Init() error
 	Open(ctx context.Context, readOnly bool) error