Refactor: libpacemaker: move pcmk__log_transition_summary() and stage8()

... from pcmk_sched_allocate.c to pcmk_graph_producer.c, since stage8() is
about creating the transition graph. pcmk__log_transition_summary() is
debatable, but it uses a static global also needed by stage8(), and it didn't
seem worth adding an accessor for that.