“In so far as impecuniosity may have a continuing relevance, it is not on the ground that the claimant lacks apparent means to satisfy any judgment, but on the ground (where this applies) that the effect of the impecuniosity would be either (i) to preclude or hinder or add to the burden of enforcement abroad against such assets as do exist abroad, or (ii) as a practical matter, to make it more likely that the claimant would take advantage of any available opportunity to avoid or hinder such enforcement abroad.”