Roundrobin

Changes to enable serving behind DNS round robin and other DNS setups involving a non 1-to-1 mapping from DNS names to IPs.  Proxies that want to take advantage of this change need to specify a Host field in the Proxy struct.  The client NewRequest is changed to take a (possibly "") `host` string parameter.  If provided, it must be used to create the new request.
Before removing this I'd get the occasional freeze while serving an HTTPS request.  I'm not sure this fixed that, though.