Moves Notifier over to handles (#97)

Internally, the linked list now uses shared_ptrs instead of raw
pointers. In addition, in the WPILib the notifier handle is now made
atomic. Then before the class is destructed, the handle is now set to 0.
This should help solve one of the existing race conditions. A 0 handle
is correctly handled down at the HAL level.

wpilibc/athena/src/Notifier.cpp

@@ -31,7 +31,9 @@ Notifier::Notifier(TimerEventHandler handler) {
  */
 Notifier::~Notifier() {
   int32_t status = 0;
-  cleanNotifier(m_notifier, &status);
+  // atomically set handle to 0, then clean
+  HalNotifierHandle handle = m_notifier.exchange(0);
+  cleanNotifier(handle, &status);
   wpi_setErrorWithContext(status, getHALErrorMessage(status));
 
   /* Acquire the mutex; this makes certain that the handler is not being