Variable server restart issues

Added mutexes in the variable server listener thread and each variable server thread.
During checkpoint restart all of these mutexes are locked by the master thread to
stop accepting new connections and stop all read/writing to all variable server clients.
Communication is resumed after the checkpoint has been reloaded.

refs #168

trick_source/sim_services/VariableServer/VariableServerThread.cpp

@@ -48,6 +48,7 @@ Trick::VariableServerThread::VariableServerThread(TCDevice * in_listen_dev) :
     connection.error_handler->report_level = TRICK_ERROR_CAUTION;
 
     pthread_mutex_init(&copy_mutex, NULL);
+    pthread_mutex_init(&restart_pause, NULL);
 
     var_data_staged = false;
     packets_copied = 0 ;