Bugfix. Blocking on full queue in sub handler causes disconnect.

If a client-side subscription channel is full, the 'Handle' RPC
call will hang indefinitely, ultimately causing the connection to
drop.  We fix this by making subscription handlers non-blocking,
ie.:  subscription messages are dropped if the consumer is not
receiving them fast enough.  This is consistent with the behavior
of libp2p-pubsub.

pkg/cap/pubsub/client.go

@@ -92,17 +92,16 @@ func (h handler) Shutdown() {
 	h.release()
 }
 
-func (h handler) Handle(ctx context.Context, call api.Topic_Handler_handle) error {
+func (h handler) Handle(_ context.Context, call api.Topic_Handler_handle) error {
 	b, err := call.Args().Msg()
 	if err != nil {
 		return err
 	}
 
 	select {
 	case h.ms <- b:
-		return nil
-
-	case <-ctx.Done():
-		return ctx.Err()
+	default:
 	}
+
+	return nil
 }