logger: add -p <topic> option to poll on a topic instead of running at fixed rate

this will be needed for fast replay. In addition, this option disables
the orb interval.
It can be removed again once we have time simulation.

src/modules/logger/logger.h

@@ -82,8 +82,8 @@ struct LoggerSubscription {
 class Logger
 {
 public:
-	Logger(LogWriter::Backend backend, size_t buffer_size, uint32_t log_interval, bool log_on_start,
-	       bool log_until_shutdown, bool log_name_timestamp, unsigned int queue_size);
+	Logger(LogWriter::Backend backend, size_t buffer_size, uint32_t log_interval, const char *poll_topic_name,
+	       bool log_on_start, bool log_until_shutdown, bool log_name_timestamp, unsigned int queue_size);
 
 	~Logger();
 
@@ -252,6 +252,7 @@ private:
 	Array<LoggerSubscription, MAX_TOPICS_NUM>	_subscriptions;
 	LogWriter					_writer;
 	uint32_t					_log_interval;
+	const orb_metadata				*_polling_topic_meta = nullptr; ///< if non-null, poll on this topic instead of sleeping
 	param_t						_log_utc_offset;
 	orb_advert_t					_mavlink_log_pub = nullptr;
 	uint16_t					_next_topic_id = 0; ///< id of next subscribed ulog topic