AudioServer: Add a buffer queue so we can buffer some sound.

The idea here is to keep a small number of sample buffers queued in the
AudioServer so we don't get caught without something to play.

Userland/aplay.cpp

@@ -25,7 +25,7 @@ int main(int argc, char **argv)
             break;
         }
         printf("Playing %d sample(s)\n", samples->sample_count());
-        a_conn.play(*samples, true);
+        a_conn.enqueue(*samples);
     }
 
     printf("Exiting! :)\n");