ReplicatedPG: create ObjectContext with SharedPtrRegistry
All new ObjectContext are replaced with calls to
SharedPtrRegistry::lookup_or_create to ensure that they are all
registered. Because the constructor is invoked with no argument, care
is taken to always initialize the destructor_callback data member
immediately afterwards.
ReplicatedPG::get_object_context contains a redundant call to
get_snapset_context that is removed.