Allow re-entrant calls when settling #13
Description
We introduced the per-rail lock to allow re-entrant calls when settling rails, but we're not currently using it for that. I don't think this is as simple as swapping noRailModificationInProgress for nonReentrant, but maybe it is? Once we've settled lockup, the payer's funds are guaranteed to be available to us to transfer (the current rail cannot be modified).