Instead of endpoints being a flat list, it is now a list of "subsets"
where each is a struct of {Addresses, Ports}.  To generate the list of
endpoints you need to take union of the Cartesian products of the
subsets.  This is compact in the vast majority of cases, yet still
represents named ports and corner cases (e.g. each pod has a different
port number).

This also stores subsets in a deterministic order (sorted by hash) to
avoid spurious updates and comparison problems.

This is a fully compatible change - old objects and clients will
keepworking as long as they don't need the new functionality.

This is the prep for multi-port Services, which will add API to produce
endpoints in this new structure.