Move PDP measurement fetching to a separate thread

Stupid CAN messages are too slow for being inline with the normal
path...

Change-Id: I160a24eb3216fc54083df576555b5bcad062ceb8
diff --git a/frc971/wpilib/wpilib_interface.h b/frc971/wpilib/wpilib_interface.h
index 7d4b92d..82936ec 100644
--- a/frc971/wpilib/wpilib_interface.h
+++ b/frc971/wpilib/wpilib_interface.h
@@ -3,16 +3,16 @@
 
 #include <stdint.h>
 
-#include "PowerDistributionPanel.h"
-
 class DriverStation;
 
 namespace frc971 {
 namespace wpilib {
 
+class PDPFetcher;
+
 // Sends out a message on ::aos::robot_state.
 void SendRobotState(int32_t my_pid, DriverStation *ds,
-                    PowerDistributionPanel *pdp);
+                    PDPFetcher *pdp_fetcher);
 
 }  // namespace wpilib
 }  // namespace frc971