There are cases where a rebalance improves dispersion, but doesn't
improve balance. This is because the balance of a ring builder is
taken to be the balance of its least-balanced device, so if there's a
device that has no partitions, wants some, but can't get them, then
we'll never save the ring builder even if every other device in the
ring got better.
We can detect this situation by looking at the dispersion number; if it
changes, then the rebalance needs to be saved in order to continue to
make progress.
Partial-Bug: #1697543
Change-Id: Ie239b958fc7e0547ffda2bebf61546bd4ef3d829
aa82d2cba8