strand: support them in type-erased signal connections

Change-Id: Id4a6bfb6c2dbd74391bac49242c849795820ade3
Reviewed-on: http://gerrit.aldebaran.lan/47346
Tested-by: gerrit
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Tested-by: pdaouadi <pdaouadi@aldebaran-robotics.com>