- use GDBusConnection instead of GDBusProxy.

- rename "call-id" to "conf-id". It's really not a "call" but
  configuration that gets added and NMPacrunnerManager ensures that
  the configuration is send to pacrunner.

- let "conf-id" keep a reference to NMPacrunnerManager. For one,
  when we remove configurations we need to call DestroyProxyConfiguration
  to remove it again. We cannot just abort the requests but must linger
  around until our configuration is properly cleaned up. Hence, we
  anyway cannot destroy the NMPacrunnerManager earlier.
  With respect to fixing shutdown not to leak anything, this merely
  means that we must wait (and iterate the main loop) as long as
  NMPacrunnerManager singleton still exits (that is anyway the plan
  how to fix shutdown).
  With these considerations it's also clear that our D-Bus calls must
  have a stricter timeout: NM_SHUTDOWN_TIMEOUT_MS.
  This is also nice because nm_pacrunner_manager_remove() no longer
  needs a manager parameter, it can just rely on having a reference
  to the manager.

- for logging the configuration IDs, don't log pointer values.
  Logging pointer values should be avoided as it defeats ASLR.
  Instead, give them a "log_id" number.

- pacrunner is a D-Bus activatable service. D-Bus activatable services
  needs special care. We don't want to start it over and over again.
  Instead, we only try to "StartServiceByName" if

    - we have any configuration to add

    - if pacrunner is currently confirmed not to be running (by watching
      name owner changes)

    - we didn't try to start it already. That means, only start it
      at the beginning and afterwards set a flag to block it. When
      we see pacrunner appear on D-Bus we always clear that flag,
      that means if pacrunner drops of, we will try to restart it
      (once).