ReplicatedPG: use hobject_t for snapset_contexts map

Otherwise, two objects with different namespaces but
the same object_t will end up clobbering each other's
contexts.

Fixes: #7634
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1398 from ceph/wip-7634

ReplicatedPG: use hobject_t for snapset_contexts map

Reviewed-by: Sage Weil <sage@inktank.com>