[#539] getsvc: Use buffer to assemble object

To reduce memory consumption.

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

pkg/services/object/get/get_test.go

@@ -629,6 +629,7 @@ func TestGetRemoteSmall(t *testing.T) {
 
 		*c1, *c2 = *c2, *c1
 
+		w.SetPayloadBuffer(make([]byte, 0))
 		err = svc.Get(ctx, p)
 		require.NoError(t, err)
 		require.Equal(t, obj, w.Object())