Determine final NameResolver to LoadBalancer communication

[ejona86]

Alluded to in #4137 (comment) . 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

[ejona86]

This issue is no longer impacted by the design of proxy information.

@zhangkun83 and I discussed how proxy will be handled. We feel good about having a ProxySocketAddress that wraps a SocketAddress (typically an InetSocketAddress). We're also comfortable doing this strategy in other cases where the extra information is required to properly use the address.

The case that isn't handled well is when this additional information is optional, because that forces all downstream consumers of the FooSocketAddress to understand it, which isn't really acceptable. Right now, we believe "this case is unlikely" because each address in an EAG is supposed to be pretty equivalent (like two addresses to the same host) and it seems really weird to have optional information at this level. If that turns out to be incorrect, we could introduce an OptionalInfoSocketAddress interface that has a getNestedSocketAddress() which would allow downstream consumers to unwrap the address independent of whatever new optional information is deemed necessary. That would be sub-optimal, but quite good as a fall-back when our design assumptions fail.

The "each address in an EAG is supposed to be pretty equivalent" is a false assumption today, as PickFirstLoadBalancer merges multiple EAGs into a single EAG to pass to LoadBalancer.Helper.createSubchannel(EquivalentAddressGroup addrs, Attributes attrs). This will be solved by avoiding the merge with a new method for the purpose: LoadBalancer.Helper.createSubchannel(List<EquivalentAddressGroup> addrs, Attributes attrs).