Rehashing doesn't rebalance keys for which the old owners and the new owners are disjoint sets

The node that does the push is computed as the last owner in the old CH that is still an owner in the new CH.
Instead it should be the last owner in the old CH that is still alive in the new CH.

With the default CH this can never happen, but with the topology-aware CH it could, e.g.:
old CH = {A

{mid=m0}, B{mid=m1}, C{mid=m1}, D{mid=m2}}
old owners(k) = {B, D}

new CH = {A{mid=m0}

, C

{mid=m1}

, D{mid=m2}}
new owners(k) =

{A, C}