Link costing based on average RTT (#1171)

This PR updates Ironwood to include the new RTT-based link costing and
updates `yggdrasilctl` to report the cost in `getPeers`.

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

src/core/api.go

@@ -30,6 +30,7 @@ type PeerInfo struct {
 	Coords        []uint64
 	Port          uint64
 	Priority      uint8
+	Cost          uint64
 	RXBytes       uint64
 	TXBytes       uint64
 	Uptime        time.Duration
@@ -94,6 +95,7 @@ func (c *Core) GetPeers() []PeerInfo {
 				peerinfo.Port = p.Port
 				peerinfo.Priority = p.Priority
 				peerinfo.Latency = p.Latency
+				peerinfo.Cost = p.Cost
 			}
 			peers = append(peers, peerinfo)
 		}