Pre-plan auto splines

(a) Make it so that the drivetrain automatically evicts old splines
(b) Set up auto to preplan splines at construction and after every auto.

Change-Id: I96ddb3a38947da02ad9ddc6fe933b7e85727dc18
diff --git a/frc971/control_loops/drivetrain/drivetrain.h b/frc971/control_loops/drivetrain/drivetrain.h
index 070cd03..325715f 100644
--- a/frc971/control_loops/drivetrain/drivetrain.h
+++ b/frc971/control_loops/drivetrain/drivetrain.h
@@ -144,7 +144,9 @@
 class DrivetrainLoop
     : public aos::controls::ControlLoop<Goal, Position, Status, Output> {
  public:
-  static constexpr size_t kNumSplineFetchers = 4;
+  // Note that we only actually store N - 1 splines, since we need to keep one
+  // fetcher free to check whether there are any new splines.
+  static constexpr size_t kNumSplineFetchers = 5;
 
   // Constructs a control loop which can take a Drivetrain or defaults to the
   // drivetrain at frc971::control_loops::drivetrain