gps file dump: re-implement with an uORB topic & write to the log file (#4987)

Drawbacks of the previous method: when writing to the SD card, there are
high delays in the write() call of several 100ms, every now and then. The
frequency and length of these events depend on:
- SD card
- used logger bandwidth
- bandwidth of gps data (RTCM)
Since the whole gps thread was blocked during this period, it lead to
gps timeouts and lost module.

What we do now is: publish an orb topic with queuing. This makes it async
and the logger takes care of buffering. This means it's best to:
- use high logger rate
- use large logger buffer
- reduce logger bandwith by disabling unused topics

src/modules/logger/logger.cpp

@@ -469,6 +469,7 @@ void Logger::add_default_topics()
 	add_topic("servorail_status", 200);
 	add_topic("mc_att_ctrl_status", 50);
 	add_topic("vehicle_status", 200);
+	add_topic("gps_dump"); //this will only be published if GPS_DUMP_COMM is set
 }
 
 int Logger::add_topics_from_file(const char *fname)