PG: add a requeue_op() function to complement requeue_ops().

Signed-off-by: Greg Farnum <greg@inktank.com>
ReplicatedPG: copy: conditionally requeue copy ops when cancelled

We may need to requeue copy ops which are cancelled as part of an acting
set change but don't change the primary. To support this, add a
"requeue" flag to cancel_copy_ops() and copy_ops(), as well as to
CopyResults. The CopyCallback is then responsible for requeuing (the
higher layers can't do so as they can't know which request actually
triggered the copy).

Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #746 from ceph/wip-6582

Wip 6582

Reviewed-by: Sage Weil <sage@inktank.com>