tor/changes/ticket25376_25762

  o Major feature (main loop, CPU usage):
    - Previously, tor would enable at startup all possible main loop event
      regardless if it needed them. For instance, directory authorities
      callbacks were fired up even for client only. We have now refactored this
      whole interface to only enable the appropriate callbacks depending on what
      are tor roles (client only, relay, hidden service, etc.). Furthermore,
      these events now depend on DisableNetwork or the hibernation state in
      order to enable them. This is a big step towards reducing client CPU usage
      by reducing the amount of wake ups the daemon does. Closes ticket 25376
      and 25762.