Reset reference count when destroying logger (issue microsoft#38)

src/loggers/bt_cout_logger.cpp

@@ -35,6 +35,7 @@ void StdCoutLogger::callback(Duration timestamp, const TreeNode& node, NodeStatu
 void StdCoutLogger::flush()
 {
     std::cout << std::flush;
+	ref_count = false;
 }
 
 }   // end namespace

src/loggers/bt_minitrace_logger.cpp

@@ -24,6 +24,7 @@ MinitraceLogger::~MinitraceLogger()
 {
     minitrace::mtr_flush();
     minitrace::mtr_shutdown();
+	ref_count = false;
 }
 
 void MinitraceLogger::callback(Duration /*timestamp*/,

src/loggers/bt_zmq_publisher.cpp

@@ -86,6 +86,7 @@ PublisherZMQ::~PublisherZMQ()
     }
     flush();
     delete zmq_;
+	ref_count = false;
 }