Dataflash: emit io thread warnings much less frequently

libraries/DataFlash/DataFlash_File.cpp

@@ -186,7 +186,12 @@ bool DataFlash_File::log_exists(const uint16_t lognum) const
 void DataFlash_File::periodic_1Hz(const uint32_t now)
 {
     if (!io_thread_alive()) {
-        GCS_MAVLINK::send_statustext_all(MAV_SEVERITY_CRITICAL, "No IO Thread Heartbeat");
+        if (io_thread_warning_decimation_counter == 0) {
+            GCS_MAVLINK::send_statustext_all(MAV_SEVERITY_CRITICAL, "No IO Thread Heartbeat");
+        }
+        if (io_thread_warning_decimation_counter++ > 57) {
+            io_thread_warning_decimation_counter = 0;
+        }
         // If you try to close the file here then it will almost
         // certainly block.  Since this is the main thread, this is
         // likely to cause a crash.

libraries/DataFlash/DataFlash_File.h

@@ -85,6 +85,7 @@ private:
 
     uint32_t _io_timer_heartbeat;
     bool io_thread_alive() const;
+    uint8_t io_thread_warning_decimation_counter;
 
     uint16_t _cached_oldest_log;