Changed netstats (packet loss) handling to script-land.

- Removed the net_stats_update event.
- Created a net_stats function for building and retrieving the
  current network statistics.
- Removed the internal timer for firing the net_stats_update event
  along with the global heartbeat_interval variable.
- Updated the netstats script to use the new BiF.
- Updated the stats script to use the new BiF.

src/Sessions.h

@@ -105,9 +105,6 @@ public:
 	// that are still active.
 	void Drain();
 
-	// Called periodically to generate statistics reports.
-	void HeartBeat(double t);
-
 	void GetStats(SessionStats& s) const;
 
 	void Weird(const char* name,