ReplicatedPG::make_writeable: fill in ssc on clone

ssc should always be filled in on live ObjectContexts.

Fixes: #7835
Signed-off-by: Samuel Just <sam.just@inktank.com>
debug ssc->head_exists crash
ReplicatedPG::do_op: more debug output around the head_exists assert

Signed-off-by: Samuel Just <sam.just@inktank.com>
fix debug assert
Merge remote-tracking branches 'origin/wip-sam-testing2', 'origin/wip-7870' and 'origin/wip-7835' into wip-sam-testing