setdiscovery: fix hang when #heads>200 (issue2971) When setting up the next sample, we always add all of the heads, regardless of the desired max sample size. But if the number of heads exceeds this size, then we don't add any more nodes from the still undecided set. (This is debatable per se, and I'll investigate it, but it's how we designed it at the moment.) The bug was that we always added the overall heads, not the heads of the remaining undecided set. Thus, if #heads>200 (desired sample size), we did not make progress any longer.