drivers/heater: add logging and minor improvements

- new heater_status logging message
 - run continously at low rate until configured sensor is found
 - fix px4io fd bugs (fd open/close/ioctl must be same thread)

src/modules/logger/logged_topics.cpp

@@ -62,6 +62,7 @@ void LoggedTopics::add_default_topics()
 	add_topic("cpuload");
 	add_topic("esc_status", 250);
 	add_topic("generator_status");
+	add_topic("heater_status");
 	add_topic("home_position");
 	add_topic("hover_thrust_estimate", 100);
 	add_topic("input_rc", 500);