osd: pull requeued requests off one at a time

osd: pull requeued requests off one at a time

Pull items off the finished queue on at a time.  In certain cases, an
event may result in new items betting added to the finished queue that
will be put at the *front* instead of the back.  See latest incarnation
of #2947.

Note that this is a significant changed in behavior in that we can
theoretically starve if an event keeps resulting in new events getting
generated.  Beware!

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