21413 stabilize owner

mem3:ushards returns a different selection within each zone by design. mem3_util:owner used ushards inappropriately, leading to multiple nodes believing they own the same job.

mem3_util:owner now takes the full, sorted list of live nodes (which, in the absence of node crashes and recoveries, is the same list on all nodes in the cluster), rotates it by the database name and document id then declares the calling node as the owner iff it is the head of the rotated list.