attempt improve node quality for small networks. never ping nodes immediately, not even when we don't have any nodes. The previous logic would get stuck in continuously pinging every new node immediately, because the node buffer would never get into the millions (this logic was tuned for utorrent). Instead, always honor the 15 minute ping timer, and if we don't have enough nodes in the buffer, return the last 16 nodes that we heard from (no pinging). The startup will probably be no worse than now, but the steady state should be a lot more reliable