[Reviewer: Rob] Support round-robin DNS lookup

Rob, please can you review this change to sipp to enable round-robin DNS?

Basically, we just do the getaddrinfo call every time we create a new connection.  Since we have dnsmasq installed locally, this is always a local lookup.  In fact, if you're using an IP address rather than a hostname (the normal way we run this), it's purely a local function call.

There's also a tweak to the congestion control stuff (which I added) at line 1550 - the congested check is superfluous here, and the `call_socket` variable isn't always set up correctly.