Move queue statistics to Worker

Just like the thread stats and poll stats earlier, the queue stats
are now moved to worker.

A litte refactoring still, and the polling will only work on local
data.

server/core/maxscale/poll.h

@@ -53,7 +53,8 @@ void            poll_init();
 void            poll_waitevents(int epoll_fd,
                                 int thread_id,
                                 THREAD_DATA* thread_data,
-                                POLL_STATS* pollStats,
+                                POLL_STATS* poll_stats,
+                                QUEUE_STATS* queue_stats,
                                 bool (*should_terminate)(void* data),
                                 void* data);
 void            poll_set_maxwait(unsigned int);