Dialer for dht

dht doesn't need the whole network interface, only needs a Dialer.
(much reduced surface of possible errors)

net/interface.go

@@ -48,3 +48,12 @@ type Handler srv.Handler
 
 // Service interface for network resources.
 type Service srv.Service
+
+// Dialer service that can dial to peers
+// (this is usually just a Network, but other services may not need the whole
+// thing, and thus it becomes easier to mock)
+type Dialer interface {
+
+	// DialPeer attempts to establish a connection to a given peer
+	DialPeer(peer.Peer) error
+}