Review `DCAwareRoundRobinPolicy` host distance calculation

[Gor027]

While working on RackAwarePolicy, it was noticed that DCAwareRoundRobinPolicy considers hosts without local datanceter information to be considered local. In RackAwarePolicy, only if localDc is UNSET then a host without local dc information can be considered local. One of these approaches is wrong, most probably DCAwareRoundRobinPolicy::distance should be modified.

[roydahan]

Need to check if it's still the case.