Determine final NameResolver to LoadBalancer communication

Alluded to in https://github.com/grpc/grpc-java/pull/4137#discussion_r171640085 . The biggest issue is probably how Java allows a list of lists of addresses. This is different than the other implementations. We need to come to a cross-language agreement about whether list of lists is necessary.

But we also need to design the final solution for how proxy information should be plumbed from the NameResolver through the LoadBalancer into Subchannel. That will remove `PairSocketAddress` or stabilize it (with a better name!).

CC @zhangkun83