Add DHTRouter interface

The implementation of ipfs/specs#476 suggests that content routers should support a DHT-specific operations (GetClosestPeers).

Content routers depend on routing interfaces defined in the `routing` package and some decorator interfaces defined here. So it seems like the natural place to add yet another interface for this type of router.

Author: hsanjuan

compconfig.go

@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"github.com/libp2p/go-libp2p-routing-helpers/tracing"
+	"github.com/libp2p/go-libp2p/core/peer"
 	"github.com/libp2p/go-libp2p/core/routing"
 	"github.com/multiformats/go-multihash"
 )
@@ -37,3 +38,13 @@ type ReadyAbleRouter interface {
 type ComposableRouter interface {
 	Routers() []routing.Routing
 }
+
+// A DHTRouter provides DHT-specific operations.
+type DHTRouter interface {
+	// GetClosestPeers returns the DHT closest peers to the given peer ID.
+	// If empty, it will use the content router's peer ID
+	// (self). `closerThan` (optional) forces the resulting records to be
+	// closer to `PeerID` than to `closerThan`. `count` specifies how many
+	// records to return ([1,100], with 20 as default when set to 0).
+	GetClosestPeers(ctx context.Context, pid peer.ID, closerThan peer.ID, count int) (<-chan peer.AddrInfo, error)
+}