osd: preserve ordering when throttling races with missing/degraded requeue

osd: preserve ordering when throttling races with missing/degraded requeue

When we delay an op because the op_queue is full, we can violate the op
order:

 - op1 comes in, waits because object is missing
 - op2 comes in, throttles on op queue
 - op1 is requeued (no longer missing)
 - queue drains, op2 happens
 - op1 happens

To avoid this, if we delay, requeue ourselves... after whatever else is
on the queue.

Fixes: #1490
Signed-off-by: Sage Weil <sage@newdream.net>