Respect device weights when adding replicas

Previously, any new (partition, replica) pairs created by adding
replicas were spread evenly across the whole ring. Now it respects
device weights, so more (partition, replica) pairs are placed on
higher-weight devices.

Note that this only affects partition assignment *when the replica
count is increased*. Normal adding of disks without changing the
replica count is, and was, just fine.

The cause was that the devices' parts_wanted values weren't being
updated when the replica count went up. Since adding replicas makes
more things to assign, each device's desired share should have gone
up, but it didn't. This appears to be a simple oversight on the part
of the original author (me).

Change-Id: Idab14be90fab243c1077a584396a9981a4bd8638
Merge "Respect device weights when adding replicas"