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>
ReplicatedPG::trim_object: use old_snaps for rollback

We need to rollback the old value of snaps, not the
new one.

Fixes: #7638
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge remote-tracking branches 'origin/wip-7638' and 'origin/wip-7634' into wip-sam-testing