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 while 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>
Merge pull request #1689 from ceph/wip-8091

Wip 8091

Reviewed-by: Sage Weil <sage@inktank.com>