osd/PG: keep PG alive until the final onreadable context under old ch is done

osd/PG: keep PG alive until the final onreadable context under old ch is done

The onreadable completions go through a finisher; add a final event
in that stream that keeps the PG alive while prior events flush.

flush() isn't quite sufficient since it doesn't wait for the finisher
events to flush too--only for the actual apply to have happened.

Signed-off-by: Sage Weil <sage@redhat.com>