Support multiple proxies in one process

[mithrandi]

soapproxy ends up using a fair amount of RAM when running it under pypy, running multiple processes thus isn't great if you don't need it for scaling. You should be able to batch these up, at least; eventually maybe we could use something like https://github.com/glyph/WarpedAlloy to automatically multiprocess but I don't think we'll need this any time soon.