refactor(tunnel): simplify tor implementation (#290)

Simplify interaction within the package by avoiding to have
a tor specific config. Use a Config instead.

Part of https://github.com/ooni/probe/issues/985.

internal/engine/tunnel/tunnel.go

@@ -41,7 +41,7 @@ func Start(ctx context.Context, config *Config) (Tunnel, error) {
 		return enforceNilContract(tun, err)
 	case "tor":
 		logger.Infof("starting %s tunnel; please be patient...", config.Name)
-		tun, err := torStart(ctx, config.Session)
+		tun, err := torStart(ctx, config)
 		return enforceNilContract(tun, err)
 	default:
 		return nil, errors.New("unsupported tunnel")