ReplicatedPG: register_object_context and register_snapset_context cleanup
Previously, get_object_context and get_snapset_context did not register
the resulting objects. In some cases, these objects would not get
registered and multiple copies would end up created. This caused a bug
in find_object_context where get_snapset_context could return an object
distinct from the one referenced by the object returned from
get_object_context.
Signed-off-by: Samuel Just <samuelj@hq.newdream.net>