osdc/Objecter: flag ops that have been redirected

In the future it may be helpful to know whether an op has been redirected
by the OSD.

We don't need it bad enough now to spend a feature bit to know whether this
flag is reliably set.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1891 from ceph/wip-8380-b

osdc/Objecter: flag ops that have been redirected