Improve continually_expire_circuits() to wake up when needed

Propose a solution for #158 (closed)

• Remove continually_expire_circuits().
• Create function spawn_expiration_task that lets the runtime schedule expiration task for the given circuit. This function is called in two places
  • in launch(), schedule tasks that expire new circuits at their unused_cutoff
  • in get_or_launch(), schedule tasks that expire circuits after max_dirtiness if they are not used yet.