[PATCH] Add DNS lookup step to ICAP connect

This seeks to add an async DNS lookup step to the ICAP connection setup
process.

As a side effect it adds a little bit of tcp_outgoing_address support to
ICAP. Its limited to "dst" ACL at present. So outgoing ACL selections
depending on HTTP request details wont work. Which makes sense since
this connection may be reused for multiple requests.

I've skipped the idea of using more than one IP result since there seems
to be no sane place to store multiple IPs between connect attempts. It
currently relies on ipcache MarkGood/MarkBad keeping a good usable IP at
the top of the IP set.

Amos