Exclude can fail if data is not distributed evenly

Currently, exclusion looks at the storage server with least available space and uses that ratio to calculate the total cluster usage. This can result into exclusion failing if add bunch of new storage processes to a cluster where existing (or one) SS has very high utilization, and DD hasn't caught up.
https://github.com/apple/foundationdb/blob/bc47f90aff019467bc63f184a073a97e0657b294/fdbclient/SpecialKeySpace.actor.cpp#L1036-L1044