HTTP worker should be able to listen on a domain socket as well

[abingham]

Right now we only allow HTTP workers to listen on network sockets. I principle they should also be able to listen on domain sockets. The underlying aiohttp code supports this. We'll just need to think about how to work that into our config/system.

[abingham]

I had this mostly working but abandoned it for a later time. It won't require much actual coding, though some careful thought will be needed about things like cwd when creating the sockets (i.e. since workers tend to run in temp directories.)