reworked the gyro board timing stuff

It now goes out of its way to get consistent non-jittery timings for
running the code to read sensor values so it can just keep track of USB
frame numbers to get accurate timestamps for the readings.
diff --git a/frc971/input/usb_receiver.h b/frc971/input/usb_receiver.h
index dcb7b92..509b740 100644
--- a/frc971/input/usb_receiver.h
+++ b/frc971/input/usb_receiver.h
@@ -108,7 +108,7 @@
 
   GyroBoardData data_;
 
-  uint32_t sequence_;
+  int32_t last_frame_number_, frame_number_;
 
   LibUSB libusb_;
   ::std::unique_ptr<LibUSBDeviceHandle> dev_handle_;