Domains: non sticky requests should be contained within a given domain

If we have a couple of domains, then non-sticky requests can go to any server in any domain.

It would be nice if we could at least make the domain sticky, so e.g. non-sticky requests always pick the same domain, as long as there are servers in that domain.

Maybe this should be a new flag, e.g. stickyDomain. If stickySession is false, then we'd check stickyDomain.