osdc/Objecter: fix invalid read of Op

...after giving up session lock in _op_submit we may no
longer touch Op* pointer, it can be freed in another thread
by osd op reply handler.

Signed-off-by: John Spray <john.spray@redhat.com>
osdc/Objecter: more clean teardown

Signed-off-by: John Spray <john.spray@redhat.com>