Avoid blocking in notification handlers.

This isn't the correct fix but the correct fix is probably not worth it.

introduces #70

notif.go

 package dht
 
 import (
+	"context"
 	"io"
+	"time"
 
 	inet "github.com/libp2p/go-libp2p-net"
 	ma "github.com/multiformats/go-multiaddr"
 	mstream "github.com/multiformats/go-multistream"
 )
 
+// TODO: There is a race condition here where we could process notifications
+// out-of-order and incorrectly mark some peers as DHT nodes (or not DHT nodes).
+// The correct fix for this is nasty so I'm not really sure it's worth it.
+// Incorrectly recording or failing to record a DHT node in the routing table
+// isn't a big issue.
+
+const dhtCheckTimeout = 10 * time.Second
+
 // netNotifiee defines methods to be used with the IpfsDHT
 type netNotifiee IpfsDHT
 
@@ -23,11 +33,21 @@ func (nn *netNotifiee) Connected(n inet.Network, v inet.Conn) {
 	default:
 	}
 
+	go func() {
+
 		// Note: We *could* just check the peerstore to see if the remote side supports the dht
 		// protocol, but its not clear that that information will make it into the peerstore
 		// by the time this notification is sent. So just to be very careful, we brute force this
 		// and open a new stream
-	s, err := dht.host.NewStream(dht.Context(), v.RemotePeer(), ProtocolDHT, ProtocolDHTOld)
+
+		// TODO: There's a race condition here where the connection may
+		// not be open (and we may sit here trying to connect). I've
+		// added a timeout but that's not really the correct fix.
+
+		ctx, cancel := context.WithTimeout(dht.Context(), dhtCheckTimeout)
+		defer cancel()
+		s, err := dht.host.NewStream(ctx, v.RemotePeer(), ProtocolDHT, ProtocolDHTOld)
+
 		switch err {
 		case nil:
 			s.Close()
@@ -42,6 +62,7 @@ func (nn *netNotifiee) Connected(n inet.Network, v inet.Conn) {
 			// real error? thats odd
 			log.Errorf("checking dht client type: %s", err)
 		}
+	}()
 }
 
 func (nn *netNotifiee) Disconnected(n inet.Network, v inet.Conn) {
@@ -51,7 +72,7 @@ func (nn *netNotifiee) Disconnected(n inet.Network, v inet.Conn) {
 		return
 	default:
 	}
-	dht.routingTable.Remove(v.RemotePeer())
+	go dht.routingTable.Remove(v.RemotePeer())
 }
 
 func (nn *netNotifiee) OpenedStream(n inet.Network, v inet.Stream) {}