src: Refactoring common.hpp and minimizing exposed includes

Face class has relatively large rewrite to hide internals using
Implementor pattern.  As of this commit, <boost/asio.hpp> is not
automatically included whenever ndn-cxx/face.hpp is included.  If it is
needed to directly work with io_service, asio.hpp should be specifically
included.

Change-Id: Ie742b851025b4e3da634eb981319df0f42937855
diff --git a/src/util/scheduler.hpp b/src/util/scheduler.hpp
index 89892e1..b2a7831 100644
--- a/src/util/scheduler.hpp
+++ b/src/util/scheduler.hpp
@@ -16,6 +16,8 @@
 #include "../common.hpp"
 #include "monotonic_deadline_timer.hpp"
 
+#include <set>
+
 namespace ndn {
 
 struct EventIdImpl; ///< \brief Private storage of information about the event