osd: requeue dup ops inline with in-progress ops

osd: requeue dup ops inline with in-progress ops

We should requeue the dups along with the originals.  This avoids
situations where, after requeue, the dups are reordered with respect to
each other.  For example:

 - client sends A, B, C
 - osd receives A
 - connection drops
 - client sends A', B', C'
 - osd puts A' in waiting_for_ondisk, starts B' and C'
 - on_change() requeues everything

Final queue order (before this patch) is
    A, B', C', A'

After this patch, the resulting queue order is
    A, A', B', C'

Or somewhat more generally, it might be:

    A, A', B, B', B'', C', C'', D'', ....

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