Add spline management to base_autonomous_actor

We can now start and run splines.  And it works!

Change-Id: Ic58cf8fb298538bd5446f00c2989f817561f9db1
diff --git a/frc971/control_loops/drivetrain/splinedrivetrain.h b/frc971/control_loops/drivetrain/splinedrivetrain.h
index c9ebbe4..c165693 100644
--- a/frc971/control_loops/drivetrain/splinedrivetrain.h
+++ b/frc971/control_loops/drivetrain/splinedrivetrain.h
@@ -99,6 +99,7 @@
   ::std::unique_ptr<Trajectory> past_trajectory_;
   ::std::unique_ptr<Trajectory> future_trajectory_;
   int32_t future_spline_idx_ = 0;  // Current spline being computed.
+  ::std::atomic<int32_t> planning_spline_idx_{-1};
 
   // TODO(alex): pull this out of dt_config.
   const ::Eigen::DiagonalMatrix<double, 5> Q =