osd: explicitly requeue waiting_for_map in on_change()

osd: explicitly requeue waiting_for_map in on_change()

Since we are requeuing stuff anyway, do it all in the correct order. This
fixes a bug where take_waiters() comes along later (at activate_map time)
and puts waiting_for_map events at the front of the queue, in front of
e.g. waiting_for_missing.  This breaks ordering from the client's
perspective.

The convention should be: whenever you requeue, requeuing everything
that logically follows it first.

Fixes: #2947
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>