Use ActorSelection instead of ActorRef in Leader

Similar to #98 , dump ActorRef resolution for ActorSelection, but this time in the context of the elected leader in the Leader actor. At first, using a resolved reference seems like a good idea in the Leader -- we care whether there is an actor alive at the other end. However, we're also doing our own liveness testing for the elected leader, and doubling up on that just leads to confusion.

Also, while we care whether there is a live remote leader, we _don't_ care which instantiation it is -- and stale ActorRefs with old ids can be problematic.

Start by locating the Leader actor and read the related issue #98 for the preceding change. The work is done when Leader uses ActorSelection for the elected leader while retaining its own liveness testing and avoiding dependence on stale ActorRef identities.

Written by the indexing model from the issue text.