poke: wait indefinitely if --timeout is not specified

Change-Id: I0051e65d27780d8b58a0c0febf3572b0c6a23e8c
diff --git a/tools/peek/ndnpoke/ndnpoke.hpp b/tools/peek/ndnpoke/ndnpoke.hpp
index e47da81..6375512 100644
--- a/tools/peek/ndnpoke/ndnpoke.hpp
+++ b/tools/peek/ndnpoke/ndnpoke.hpp
@@ -48,7 +48,7 @@
 
   // program behavior options
   bool wantForceData = false;
-  time::milliseconds timeout = 10_s;
+  optional<time::milliseconds> timeout;
 };
 
 class NdnPoke : boost::noncopyable