ReplicatedPG::get_snapset_context: assert snap obj is not missing

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::recover_replicas: do not recover clones snap obj is missing

Otherwise, we cannot safely read the snapset for the clone.

Fixes: #8091
Signed-off-by: Samuel Just <sam.just@inktank.com>