Use MapAddress() early on, so that conn->ipaddr is never a v4-to-v6 mapped

By calling MapAddress() in the main loop (inside StartServer()) we avoid
the need for calling MapAddress tens of times later. We lose the
information that 192.168.1.2 might originate from ::ffff:192.168.1.2,
but shouldn't matter, all access controls in that case are written in
pure v4 style anyway.