dht/query: make sure to cancel all contexts.

We are leaking peer queues:

http://gateway.ipfs.io/ipfs/QmQxVA48CzVwwNYExUiFe56VrUBn8u368ZfchnCLoc7fSC/moriarty

routing/dht/query.go

@@ -52,6 +52,9 @@ type queryFunc func(context.Context, peer.ID) (*dhtQueryResult, error)
 
 // Run runs the query at hand. pass in a list of peers to use first.
 func (q *dhtQuery) Run(ctx context.Context, peers []peer.ID) (*dhtQueryResult, error) {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
 	runner := newQueryRunner(ctx, q)
 	return runner.Run(peers)
 }