Make RAV value threshold per sender

[aasseman]

          That's a good point. Is our thinking so far that the RAV request threshold is per `(sender, allocation)`? From a user (i.e. indexer) perspective, that doesn't feel very intuitive. More intuitive would indeed be: "per sender, I'm willing to run a risk of X GRT remaining uncollectable". That would also open the door to per-sender risk thresholds.

Originally posted by @Jannis in #83 (comment)