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.h

@@ -38,8 +38,9 @@ enum TimerType : uint8_t {
 	TIMER_TRIGGER,
 	TIMER_PPID_CHECK,
 	TIMER_TIMERMGR_EXPIRE,
+	TIMER_THREAD_HEARTBEAT,
 };
-const int NUM_TIMER_TYPES = int(TIMER_TIMERMGR_EXPIRE) + 1;
+const int NUM_TIMER_TYPES = int(TIMER_THREAD_HEARTBEAT) + 1;
 
 extern const char* timer_type_to_string(TimerType type);