face: making Face::shutdown call asynchronous

After Face::processInterestFilters changed to dispatch all callbacks
that match the configured filters, calling shutdown() from inside the
callback caused segfault (unexpected invalidation of the iterator).

Change-Id: Ia0a3048ce3574d354071e091c2d9400ad9a263b4
Refs: #1366
diff --git a/src/face.hpp b/src/face.hpp
index 4cb4c67..be1ca06 100644
--- a/src/face.hpp
+++ b/src/face.hpp
@@ -227,6 +227,8 @@
   void 
   onReceiveElement(const Block& wire);
 
+  void
+  asyncShutdown();
   
   static void
   fireProcessEventsTimeout(const boost::system::error_code& error);