tevent: add tevent_queue_add_entry() and tevent_queue_add_optimize_empty()

tevent: add tevent_queue_add_entry() and tevent_queue_add_optimize_empty()

This adds more flexible handling for the add operation:

- It allows the caller to remove a tevent_req from the queue
  by calling talloc_free() on the returned tevent_queue_entry.
- It allows the caller to optimize for the empty queue case,
  where it the caller wants to avoid the delay caused by
  the immediate event.

metze