add skynet.stat() to get cpu cost and message count

skynet-src/skynet_timer.h

@@ -6,6 +6,7 @@
 int skynet_timeout(uint32_t handle, int time, int session);
 void skynet_updatetime(void);
 uint32_t skynet_starttime(void);
+uint64_t skynet_thread_time(void);	// for profile, in micro second
 
 void skynet_timer_init(void);