Correctly stop link_manager sender and receiver threads

This changes the destroy_link_manager() function to:
* call a new function message_get_unlock() to unlock the sending thread from
  its blocking message_get()
* calls pthread_cancel() on the receiving thread, because this unlocks any
  blocking read from a file descriptor

The same message_get_unlock() function has been added ringbuffer_queue to keep
the interfaces the same (it does nothing, because ringbuffer_queue is
non-blocking).

openair2/UTIL/ASYNC_IF/ringbuffer_queue.c

@@ -119,6 +119,11 @@ int message_get(message_queue_t *queue, void **data, int *priority) {
   return size;
 }
 
+void message_get_unlock(message_queue_t *queue) {
+  /* don't do anything, this function exists to unlock a message_queue but is
+   * not needed in the case of the ringbuffer_queue */
+}
+
 void destroy_message_queue(message_queue_t *queue) {
   struct lfds700_misc_prng_state ls;