Rearrangements to the receive thread.

Magnitude conversion now happens immediately when sample data is
received, so there is no risk of newly received data clobbering old
data under CPU overload.

stats.h

@@ -63,8 +63,8 @@ struct stats {
     // Mode A/C demodulator counts:
     uint32_t demod_modeac;
 
-    uint32_t blocks_processed;
-    uint32_t blocks_dropped;
+    uint64_t samples_processed;
+    uint64_t samples_dropped;
 
     // timing:
     struct timespec demod_cpu;