Change how Dispatcher is shut down.

The previous use of a timeout resulting in thread detach instead of thread
join resulted in a race condition on Mac between destruction and thread
closeout.  This commit removes the detach functionality and uses dup2() to
on Linux/Mac and connecting to itself on Windows to try to ensure accept()
exits.

src/tcpsockets/TCPAcceptor.h

@@ -24,6 +24,7 @@
 #ifndef TCPSOCKETS_TCPACCEPTOR_H_
 #define TCPSOCKETS_TCPACCEPTOR_H_
 
+#include <atomic>
 #include <memory>
 #include <string>
 
@@ -35,7 +36,7 @@ class TCPAcceptor : public NetworkAcceptor {
   int m_port;
   std::string m_address;
   bool m_listening;
-  bool m_shutdown;
+  std::atomic_bool m_shutdown;
 
  public:
   TCPAcceptor(int port, const char* address);