Measure CPU for demodulator, read thread, and background tasks individually.

dump1090.h

@@ -289,6 +289,7 @@ struct {                             // Internal state
     int             iDataOut;        // Fifo output pointer
     int             iDataReady;      // Fifo content count
     int             iDataLost;       // Count of missed buffers
+    struct timespec reader_cpu_accumulator; // CPU time used by the reader thread, copied out and reset by the main thread under the mutex
 
     int             trailing_samples;// extra trailing samples in magnitude buffer