Resolves #405

Adds 500 ms delay to initial registering for samples from all tuners to
feed the spectral display/waterfall.  The existing approach of immediately
registering for samples was causing the airspy tuner to lockup
aperiodically.  Investigation via debugger didn't show any application
thread locks ... usb master processor was continuing to service libusb
timeouts.  Unable to debug internal airspy processes.  Adding the delay
to startup seems to be a usable solution.

src/main/java/io/github/dsheirer/source/tuner/TunerManager.java

@@ -71,6 +71,10 @@ public class TunerManager
      */
     public void dispose()
     {
+        mLog.info("Shutting down LibUsb timeout processor ...");
+
+        LIBUSB_TRANSFER_PROCESSOR.shutdown();
+
         mLog.info("Shutting down LibUsb ...");
         try
         {