Objecter::_op_submit: only replace the tid if it's 0

Otherwise, redirected ops will suddenly have a different tid
and will become uncancelable.

Fixes: #7588
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branches 'origin/wip-sam-testing2', 'origin/wip-7942', 'origin/wip-8020' and 'upstream/wip-7891' into wip-sam-testing