Threading changes for the new loop architecture

- threading::Manager is no longer an IOSource.
- threading::MsgThread is now an IOSource. This allows threads themselves to signal when they have data to process instead of continually checking each of the threads on every loop pass.
- Make the thread heartbeat timer an actual timer and let it fire as necessary instead of checking to see if it should fire

src/Timer.cc

@@ -43,6 +43,7 @@ const char* TimerNames[] = {
 	"TriggerTimer",
 	"ParentProcessIDCheck",
 	"TimerMgrExpireTimer",
+	"ThreadHeartbeat",
 };
 
 const char* timer_type_to_string(TimerType type)