gh-145685: Stop the world when updating MRO of existing types (gh-145707)

gh-145685: Stop the world when updating MRO of existing types (gh-145707)

We already have a stop-the-world pause elsewhere in this code path
(type_set_bases) and this makes will make it easier to avoid contention
on the TYPE_LOCK when looking up names in the MRO hierarchy.

Also use deferred reference counting for non-immortal MROs.