utils/tracers: Adding *::Destroy () method in Tracers

This way it becomes possible to do clean up before applications
terminates (useful if the same scenario is used for several simulation
runs or if there is a need for post-processing of the resulting files)
diff --git a/utils/tracers/ndn-app-delay-tracer.cc b/utils/tracers/ndn-app-delay-tracer.cc
index c0f881d..6eeeeb9 100644
--- a/utils/tracers/ndn-app-delay-tracer.cc
+++ b/utils/tracers/ndn-app-delay-tracer.cc
@@ -53,6 +53,12 @@
 }
 
 void
+AppDelayTracer::Destroy ()
+{
+  g_tracers.clear ();
+}
+
+void
 AppDelayTracer::InstallAll (const std::string &file)
 {
   using namespace boost;