WIP: Services v2 (ip-per-service)

DO NOT COMMIT.  Needs test updates and a new e2e test.  Some FIXMEs still.  95% complete.

Add support for IP-per-service behavior.  This creates virtual IP addresses for each service, and uses iptables to route traffic "magically".

The first few commits are relatively independent, and could be extracted to different PRs, if you feel like making me suffer.

See #1107 for background.
We could make that optimization if we decided it was important to keep global portals, no pre-declarations, and scaling was actually a problem.  IMO it's too clever by half, but it's worth keeping in our pocket, I suppose.