Switches all notifiers created with the external API to be threaded (#546)

Testing showed this wasn't an issue with timing, and allows for more
safety in user code making mistakes. Places where the extra thread
wouldn't help have been kept non threaded, using a new internal API.

hal/include/HAL/cpp/NotifierInternal.h

@@ -0,0 +1,15 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2017. All Rights Reserved.                             */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include "HAL/Types.h"
+
+extern "C" {
+HAL_NotifierHandle HAL_InitializeNotifierNonThreadedUnsafe(
+    HAL_NotifierProcessFunction process, void* param, int32_t* status);
+}