[resolver and LB policy APIs] change address list to support multiple addresses per endpoint

[markdroth]

More changes as part of the dualstack design:

• Change resolver and LB policy APIs to support multiple addresses per endpoint. Specifically, replace ServerAddress with EndpointAddresses, which encodes more than one address. Per-address channel args are retained at the same level, so they are now per-endpoint. For now, EndpointAddress provides a single-address ctor and a single-address accessor for backward compatibility, so ServerAdress is an alias for EndpointAddresses; eventually, this alias and the single-address methods will be removed.
• Add an EndpointAddressSet class, which represents an unordered set of addresses to be used as a map key. This will be used in a number of LB policies that need to store per-endpoint state.
• Change the LB policy API's ChannelControlHelper::CreateSubchannel() method to take the address and per-endpoint channel args as separate parameters, so that we don't need to construct a legacy ServerAddress object as we create a new subchannel for each address in the endpoint.
• Change pick_first to flatten the address list.
• Change ring_hash to use EndpointAddressSet as the key for its endpoint map, and to use the first address of the endpoint as the hash key.
• Change WRR to use EndpointAddressSet as the key for its endpoint weight map.

Note that support for multiple addresses per endpoint is guarded in RR by the existing round_robin_delegate_to_pick_fist experiment and in WRR by the existing wrr_delegate_to_pick_first experiment.

This PR does not include support for multiple addresses per endpoint for the outlier_detection or xds_override_host LB policies; those will come in subsequent PRs.